diff --git a/source/background/services/autoLogin.ts b/source/background/services/autoLogin.ts
index a21a8b1f..ee31edbb 100644
--- a/source/background/services/autoLogin.ts
+++ b/source/background/services/autoLogin.ts
@@ -14,7 +14,7 @@ export function getAutoLoginForTab(tabID: number): SearchResult | null {
     const register = getRegister();
     const key = `tab-${tabID}`;
     if (register.has(key)) {
-        const item = register.get(key).entry;
+        const item = (register.get(key) as RegisteredItem).entry;
         register.delete(key);
         return item;
     }
diff --git a/source/background/services/config.ts b/source/background/services/config.ts
index aff91eda..84abd9eb 100644
--- a/source/background/services/config.ts
+++ b/source/background/services/config.ts
@@ -1,5 +1,6 @@
 import { getSyncValue, setSyncValue } from "./storage.js";
 import { Configuration, InputButtonType, SyncStorageItem } from "../types.js";
+import { naiveClone } from "../../shared/library/clone.js";
 
 const DEFAULTS: Configuration = {
     entryIcons: true,
@@ -9,7 +10,7 @@ const DEFAULTS: Configuration = {
     useSystemTheme: true
 };
 
-let __lastConfig: Configuration = null;
+let __lastConfig: Configuration | null = null;
 
 export function getConfig(): Configuration {
     if (!__lastConfig) {
@@ -24,7 +25,7 @@ export async function initialise() {
 
 export async function updateConfigValue<T extends keyof Configuration>(key: T, value: Configuration[T]): Promise<void> {
     const configRaw = await getSyncValue(SyncStorageItem.Configuration);
-    const config = JSON.parse(configRaw);
+    const config = configRaw ? JSON.parse(configRaw) : naiveClone(DEFAULTS);
     config[key] = value;
     __lastConfig = config;
     await setSyncValue(SyncStorageItem.Configuration, JSON.stringify(config));
diff --git a/source/background/services/desktop/actions.ts b/source/background/services/desktop/actions.ts
index 411dd2fc..520e8381 100644
--- a/source/background/services/desktop/actions.ts
+++ b/source/background/services/desktop/actions.ts
@@ -85,7 +85,7 @@ export async function getVaultsTree(): Promise<VaultsTree> {
             ...output,
             [sourceID]: {
                 ...tree[sourceID],
-                name: names[sourceID] ?? "Untitled vault"
+                name: names?.[sourceID] ?? "Untitled vault"
             }
         };
     }, {});
diff --git a/source/background/services/desktop/request.ts b/source/background/services/desktop/request.ts
index d4351d32..3784fa19 100644
--- a/source/background/services/desktop/request.ts
+++ b/source/background/services/desktop/request.ts
@@ -5,7 +5,7 @@ import { decryptPayload, encryptPayload } from "../crypto.js";
 import { getLocalValue } from "../storage.js";
 import { LocalStorageItem } from "../../types.js";
 
-type OutputType = "body" | "status";
+type OutputType = "body" | "status" | undefined;
 
 interface DesktopRequestConfig<O extends OutputType> {
     auth?: string | null;
@@ -45,12 +45,13 @@ export async function sendDesktopRequest<O extends OutputType>(
             url = newURL.toString();
         } else {
             requestConfig.body = JSON.stringify(payload);
-            Object.assign(requestConfig.headers, {
+            Object.assign(requestConfig.headers as HeadersInit, {
                 "Content-Type": "application/json"
             });
         }
     }
     if (auth !== null) {
+        requestConfig.headers = requestConfig.headers || {};
         // Request requires encryption, perform setup now
         requestConfig.headers["Authorization"] = auth;
         if (typeof requestConfig.body === "string") {
@@ -59,6 +60,12 @@ export async function sendDesktopRequest<O extends OutputType>(
             // Encrypt
             const privateKey = await getLocalValue(LocalStorageItem.APIPrivateKey);
             const publicKey = await getLocalValue(LocalStorageItem.APIServerPublicKey);
+            if (!privateKey) {
+                throw new Error("Authenticated request failed: No private key available");
+            }
+            if (!publicKey) {
+                throw new Error("Authenticated request failed: No public key available");
+            }
             requestConfig.body = await encryptPayload(requestConfig.body, privateKey, publicKey);
         }
     }
@@ -81,19 +88,25 @@ export async function sendDesktopRequest<O extends OutputType>(
     }
     // Handle encrypted response
     if (resp.headers.get("X-Bcup-API")) {
-        const components = resp.headers.get("X-Bcup-API").split(",");
+        const components = resp.headers.get("X-Bcup-API")?.split(",") ?? [];
         if (components.includes("enc")) {
             const content = await resp.text();
             const contentType = resp.headers.get("X-Content-Type") || resp.headers.get("Content-Type") || "text/plain";
             // Decrypt
             const privateKey = await getLocalValue(LocalStorageItem.APIPrivateKey);
             const publicKey = await getLocalValue(LocalStorageItem.APIServerPublicKey);
+            if (!privateKey) {
+                throw new Error("Decrypting response failed: No private key available");
+            }
+            if (!publicKey) {
+                throw new Error("Decrypting response failed: No public key available");
+            }
             const rawDecrypted = await decryptPayload(content, publicKey, privateKey);
             return /application\/json/.test(contentType) ? JSON.parse(rawDecrypted) : rawDecrypted;
         }
     }
     // Standard, unencrypted response
-    if (/application\/json/.test(resp.headers.get("Content-Type"))) {
+    if (/application\/json/.test(resp.headers.get("Content-Type") ?? "")) {
         return resp.json();
     }
     return resp.text();
diff --git a/source/background/services/entry.ts b/source/background/services/entry.ts
index 5ce4c59d..b085d334 100644
--- a/source/background/services/entry.ts
+++ b/source/background/services/entry.ts
@@ -4,5 +4,8 @@ import { formatURL } from "../../shared/library/url.js";
 
 export async function openEntryPageInNewTab(_: SearchResult, url: string): Promise<number> {
     const tab = await createNewTab(formatURL(url));
+    if (typeof tab.id !== "number") {
+        throw new Error("No tab ID for created tab");
+    }
     return tab.id;
 }
diff --git a/source/background/services/loginMemory.ts b/source/background/services/loginMemory.ts
index 7cff093f..1af867e8 100644
--- a/source/background/services/loginMemory.ts
+++ b/source/background/services/loginMemory.ts
@@ -17,7 +17,7 @@ export function clearCredentials(id: string): void {
     }
     if (memory.has("last")) {
         const last = memory.get("last");
-        if (last.credentials.id === id) {
+        if (last?.credentials.id === id) {
             memory.delete("last");
         }
     }
@@ -35,7 +35,7 @@ export function getAllCredentials(): Array<UsedCredentials> {
 
 export function getCredentialsForID(id: string): UsedCredentials | null {
     const memory = getLoginMemory();
-    return memory.has(id) ? memory.get(id).credentials : null;
+    return memory.has(id) ? (memory.get(id) as LoginMemoryItem).credentials : null;
 }
 
 export function getLastCredentials(tabID: number): UsedCredentials | null {
@@ -55,7 +55,7 @@ function getLoginMemory(): ExpiryMap<string, LoginMemoryItem> {
 export function stopPromptForID(id: string): void {
     const memory = getLoginMemory();
     if (memory.has(id)) {
-        const existing = memory.get(id);
+        const existing = memory.get(id) as LoginMemoryItem;
         memory.set(id, {
             ...existing,
             credentials: {
diff --git a/source/background/services/messaging.ts b/source/background/services/messaging.ts
index 43935676..ad260bfd 100644
--- a/source/background/services/messaging.ts
+++ b/source/background/services/messaging.ts
@@ -53,8 +53,12 @@ async function handleMessage(
 ) {
     switch (msg.type) {
         case BackgroundMessageType.AuthenticateDesktopConnection: {
+            const { code } = msg;
+            if (!code) {
+                throw new Error("No auth code provided");
+            }
             log("complete desktop authentication");
-            const publicKey = await authenticateBrowserAccess(msg.code);
+            const publicKey = await authenticateBrowserAccess(code);
             await setLocalValue(LocalStorageItem.APIServerPublicKey, publicKey);
             sendResponse({});
             break;
@@ -78,6 +82,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.ClearSavedCredentials: {
             const { credentialsID } = msg;
+            if (!credentialsID) {
+                throw new Error("No credentials ID provided");
+            }
             log(`clear saved credentials: ${credentialsID}`);
             clearCredentials(credentialsID);
             sendResponse({});
@@ -85,6 +92,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.ClearSavedCredentialsPrompt: {
             const { credentialsID } = msg;
+            if (!credentialsID) {
+                throw new Error("No credentials ID provided");
+            }
             log(`clear saved credentials prompt: ${credentialsID}`);
             stopPromptForID(credentialsID);
             await sendTabsMessage({
@@ -95,6 +105,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.DeleteDisabledDomains: {
             const { domains } = msg;
+            if (!domains) {
+                throw new Error("No domains list provided");
+            }
             log(`remove disabled domains: ${domains.join(", ")}`);
             for (const domain of domains) {
                 await removeDisabledFlagForDomain(domain);
@@ -104,6 +117,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.DisableSavePromptForCredentials: {
             const { credentialsID } = msg;
+            if (!credentialsID) {
+                throw new Error("No credentials ID provided");
+            }
             log(`disable save prompt for credentials: ${credentialsID}`);
             try {
                 const credentials = getCredentialsForID(credentialsID);
@@ -208,6 +224,9 @@ async function handleMessage(
             break;
         }
         case BackgroundMessageType.GetSavedCredentialsForID: {
+            if (!msg.credentialsID) {
+                throw new Error("No credentials ID provided");
+            }
             const credentials = getCredentialsForID(msg.credentialsID);
             sendResponse({
                 credentials: [credentials]
@@ -223,6 +242,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.MarkNotificationRead: {
             const { notification } = msg;
+            if (!notification) {
+                throw new Error("No notification provided");
+            }
             log(`mark notification read: ${notification}`);
             await markNotificationRead(notification);
             sendResponse({});
@@ -230,6 +252,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.OpenEntryPage: {
             const { autoLogin, entry } = msg;
+            if (!entry) {
+                throw new Error("No entry provided");
+            }
             const [url = null] = getEntryURLs(entry.properties, EntryURLType.Login);
             if (!url) {
                 sendResponse({ opened: false });
@@ -253,6 +278,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.PromptLockSource: {
             const { sourceID } = msg;
+            if (!sourceID) {
+                throw new Error("No source ID provided");
+            }
             log(`request lock source: ${sourceID}`);
             const locked = await promptSourceLock(sourceID);
             sendResponse({
@@ -262,6 +290,9 @@ async function handleMessage(
         }
         case BackgroundMessageType.PromptUnlockSource: {
             const { sourceID } = msg;
+            if (!sourceID) {
+                throw new Error("No source ID provided");
+            }
             log(`request unlock source: ${sourceID}`);
             await promptSourceUnlock(sourceID);
             sendResponse({});
@@ -276,6 +307,15 @@ async function handleMessage(
         }
         case BackgroundMessageType.SaveCredentialsToVault: {
             const { sourceID, groupID, entryID = null, entryProperties, entryType = EntryType.Website } = msg;
+            if (!sourceID) {
+                throw new Error("No source ID provided");
+            }
+            if (!groupID) {
+                throw new Error("No group ID provided");
+            }
+            if (!entryProperties) {
+                throw new Error("No entry properties provided");
+            }
             if (entryID) {
                 log(`save credentials to existing entry: ${entryID} (source=${sourceID})`);
                 await saveExistingEntry(sourceID, groupID, entryID, entryProperties);
@@ -293,31 +333,52 @@ async function handleMessage(
         }
         case BackgroundMessageType.SaveUsedCredentials: {
             const { credentials } = msg;
+            if (!credentials) {
+                throw new Error("No source ID provided");
+            }
+            if (!sender.tab?.id) {
+                throw new Error("No tab ID available for background message");
+            }
             updateUsedCredentials(credentials, sender.tab.id);
             sendResponse({});
             break;
         }
         case BackgroundMessageType.SearchEntriesByTerm: {
-            const searchResults = await searchEntriesByTerm(msg.searchTerm);
+            const { searchTerm } = msg;
+            if (!searchTerm) {
+                throw new Error("No search term provided");
+            }
+            const searchResults = await searchEntriesByTerm(searchTerm);
             sendResponse({
                 searchResults
             });
             break;
         }
         case BackgroundMessageType.SearchEntriesByURL: {
-            const searchResults = await searchEntriesByURL(msg.url);
+            const { url } = msg;
+            if (!url) {
+                throw new Error("No URL provided");
+            }
+            const searchResults = await searchEntriesByURL(url);
             sendResponse({
                 searchResults
             });
             break;
         }
         case BackgroundMessageType.SetConfigurationValue: {
-            await updateConfigValue(msg.configKey, msg.configValue);
+            const { configKey, configValue } = msg;
+            if (!configKey || typeof configValue === "undefined") {
+                throw new Error("Invalid configuration proivided provided");
+            }
+            await updateConfigValue(configKey, configValue);
             sendResponse({});
             break;
         }
         case BackgroundMessageType.TrackRecentEntry: {
             const { entry } = msg;
+            if (!entry) {
+                throw new Error("No entry provided");
+            }
             if (!entry.sourceID) {
                 throw new Error(`No source ID in entry result: ${entry.id}`);
             }
diff --git a/source/background/services/tabs.ts b/source/background/services/tabs.ts
index 27eb6b00..7059304f 100644
--- a/source/background/services/tabs.ts
+++ b/source/background/services/tabs.ts
@@ -9,10 +9,13 @@ export async function sendTabsMessage(payload: TabEvent, tabIDs: Array<number> |
               await browser.tabs.query({
                   status: "complete"
               })
-          ).map((tab) => tab.id);
+          ).reduce((output: Array<number>, tab) => {
+              if (!tab.id) return output;
+              return [...output, tab.id];
+          }, []);
     await Promise.all(
         targetTabIDs.map(async (tabID) => {
-            await browser.tabs.sendMessage(tabID, payload);
+            browser.tabs.sendMessage(tabID, payload);
         })
     );
 }
diff --git a/source/full/components/pages/NotificationsPage.tsx b/source/full/components/pages/NotificationsPage.tsx
index bed643b6..313d95cd 100644
--- a/source/full/components/pages/NotificationsPage.tsx
+++ b/source/full/components/pages/NotificationsPage.tsx
@@ -23,6 +23,7 @@ export function NotificationsPage() {
         ])]);
         setCurrentTab(newTabID);
         const key = Object.keys(NOTIFICATIONS).find(nKey => NOTIFICATIONS[nKey][0] === newTabID);
+        if (!key) return;
         updateReadNotifications(key).catch(err => {
             console.error(err);
             getToaster().show({
@@ -38,7 +39,7 @@ export function NotificationsPage() {
     }, [currentTab, handleTabChange, notifications]);
     return (
         <Layout title={t("notifications.title")}>
-            <Tabs onChange={handleTabChange} selectedTabId={currentTab}>
+            <Tabs onChange={handleTabChange} selectedTabId={currentTab ?? undefined}>
                 {notifications.map(([nameKey, Component]) => (
                     <Tab
                         key={nameKey}
diff --git a/source/full/components/pages/saveCredentials/CredentialsSaver.tsx b/source/full/components/pages/saveCredentials/CredentialsSaver.tsx
index e552aa88..50db088b 100644
--- a/source/full/components/pages/saveCredentials/CredentialsSaver.tsx
+++ b/source/full/components/pages/saveCredentials/CredentialsSaver.tsx
@@ -1,5 +1,5 @@
 import React, { Fragment, useCallback, useMemo, useState } from "react";
-import { NonIdealState, Tree, TreeNodeInfo } from "@blueprintjs/core";
+import { ITreeNode, NonIdealState, Tree, TreeNodeInfo } from "@blueprintjs/core";
 import { EntryPropertyType, VaultFacade, VaultSourceID, fieldsToProperties } from "buttercup";
 import { SiteIcon } from "@buttercup/ui";
 import styled from "styled-components";
@@ -168,20 +168,20 @@ export function CredentialsSaver(props: CredentialsSaverProps) {
                 // Only select entries
                 setSelectedNodes([node.nodeData.id]);
             }
-        } else if (mode === "new") {
+        } else if (mode === "new" && node.nodeData) {
             // Groups only, select immediately
             setSelectedNodes([node.nodeData.id]);
         }
     }, [mode, saving]);
     const handleSaveClick = useCallback((credentials: UsedCredentials) => {
-        if (mode === "new") {
+        if (mode === "new" && selectedGroupURI) {
             const [, sourceID, groupID] = selectedGroupURI.split(":");
             onSaveNewClick({
                 ...credentials,
                 groupID,
                 sourceID
             });
-        } else if (mode === "existing") {
+        } else if (mode === "existing" && selectedEntryURI) {
             const [, sourceID, groupID, entryID] = selectedEntryURI.split(":");
             onSaveNewClick({
                 ...credentials,
@@ -212,12 +212,15 @@ export function CredentialsSaver(props: CredentialsSaverProps) {
                             contents={contents}
                             onNodeClick={handleNodeClick}
                             onNodeCollapse={node => setExpandedNodes(
-                                current => current.filter(id => id !== node.nodeData.id)
+                                current => current.filter(id => id !== node.nodeData?.id)
                             )}
-                            onNodeExpand={node => setExpandedNodes(current => [
-                                ...current,
-                                node.nodeData.id
-                            ])}
+                            onNodeExpand={node => {
+                                if (!node.nodeData) return;
+                                setExpandedNodes(current => [
+                                    ...current,
+                                    (node.nodeData as NodeInfo).id
+                                ]);
+                            }}
                         />
                     ) || (
                         <NonIdealState
@@ -226,14 +229,14 @@ export function CredentialsSaver(props: CredentialsSaverProps) {
                             description={t("save-credentials-page.credentials-saver.no-vaults.description")}
                         />
                     )}
-                    {mode === "new" && contents.length > 0 && selectedGroupURI && (
+                    {mode === "new" && contents.length > 0 && selectedGroupURI && selectedUsedCredentials && (
                         <NewEntrySavePrompt
                             credentials={selectedUsedCredentials}
                             onSaveClick={handleSaveClick}
                             saving={saving}
                         />
                     )}
-                    {mode === "existing" && contents.length > 0 && selectedEntryURI && (
+                    {mode === "existing" && contents.length > 0 && selectedEntryURI && selectedUsedCredentials && (
                         <NewEntrySavePrompt
                             credentials={selectedUsedCredentials}
                             onSaveClick={handleSaveClick}
diff --git a/source/popup/components/pages/EntriesPage.tsx b/source/popup/components/pages/EntriesPage.tsx
index fc9c95fc..d5cf175d 100644
--- a/source/popup/components/pages/EntriesPage.tsx
+++ b/source/popup/components/pages/EntriesPage.tsx
@@ -96,7 +96,7 @@ function EntriesPageList(props: EntriesPageProps) {
     const urlEntries = useEntriesForURL(url);
     const recentEntries = useRecentEntries();
     const handleEntryClick = useCallback((entry: SearchResult, autoLogin: boolean) => {
-        if (popupSource === "page") {
+        if (popupSource === "page" && formID) {
             sendEntryResultToTabForInput(formID, entry);
         } else if (popupSource === "popup") {
             openPageForEntry(entry, autoLogin)
diff --git a/source/popup/components/pages/OTPsPage.tsx b/source/popup/components/pages/OTPsPage.tsx
index a56eaaab..e3bbc4bc 100644
--- a/source/popup/components/pages/OTPsPage.tsx
+++ b/source/popup/components/pages/OTPsPage.tsx
@@ -87,7 +87,7 @@ function OTPsPageList(props: OTPsPageProps) {
     const [otps, loadingOTPs] = useOTPs();
     const preparedOTPs = usePreparedOTPs(otps);
     const handleOTPClick = useCallback((otp: OTP) => {
-        if (popupSource === "page") {
+        if (popupSource === "page" && formID) {
             sendOTPToTabForInput(formID, otp);
         } else if (popupSource === "popup") {
             if (!otp.loginURL) {
diff --git a/source/popup/components/pages/SaveDialogPage.tsx b/source/popup/components/pages/SaveDialogPage.tsx
index cd5871eb..32249c11 100644
--- a/source/popup/components/pages/SaveDialogPage.tsx
+++ b/source/popup/components/pages/SaveDialogPage.tsx
@@ -107,6 +107,7 @@ export function SaveDialogPage() {
         }
     }, [loginID]);
     const handleCloseClick = useCallback(async () => {
+        if (!loginID) return;
         try {
             // Clear prompt and close dialog
             await clearSavedLoginPrompt(loginID);
@@ -120,6 +121,7 @@ export function SaveDialogPage() {
         }
     }, [loginID]);
     const handleDisableClick = useCallback(async () => {
+        if (!loginID) return;
         if (!disableConfirm) {
             setDisableConfirm(true);
             return;
diff --git a/source/popup/hooks/credentials.ts b/source/popup/hooks/credentials.ts
index 2e571b78..5b019cec 100644
--- a/source/popup/hooks/credentials.ts
+++ b/source/popup/hooks/credentials.ts
@@ -4,14 +4,14 @@ import { sendBackgroundMessage } from "../../shared/services/messaging.js";
 import { BackgroundMessageType, UsedCredentials } from "../types.js";
 import { useCallback } from "react";
 
-async function getAllCredentials(): Promise<Array<UsedCredentials>> {
+async function getAllCredentials(): Promise<Array<UsedCredentials | null>> {
     const resp = await sendBackgroundMessage({
         type: BackgroundMessageType.GetSavedCredentials
     });
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching saved credentials");
     }
-    return resp.credentials;
+    return resp.credentials ?? [];
 }
 
 async function getCredentialsForID(id: string): Promise<UsedCredentials | null> {
@@ -22,17 +22,17 @@ async function getCredentialsForID(id: string): Promise<UsedCredentials | null>
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching saved credentials");
     }
-    return resp.credentials[0] ?? null;
+    return resp.credentials?.[0] ?? null;
 }
 
-export function useAllLoginCredentials(): AsyncResult<Array<UsedCredentials>> {
+export function useAllLoginCredentials(): AsyncResult<Array<UsedCredentials | null>> {
     const getCredentials = useCallback(() => getAllCredentials(), []);
     const result = useAsync(getCredentials, [getCredentials]);
     return result;
 }
 
-export function useLoginCredentials(loginID: string): AsyncResult<UsedCredentials | null> {
-    const getCredentials = useCallback(() => getCredentialsForID(loginID), [loginID]);
+export function useLoginCredentials(loginID: string | null): AsyncResult<UsedCredentials | null> {
+    const getCredentials = useCallback(async () => (loginID ? await getCredentialsForID(loginID) : null), [loginID]);
     const result = useAsync(getCredentials, [getCredentials]);
     return result;
 }
diff --git a/source/popup/hooks/desktop.ts b/source/popup/hooks/desktop.ts
index 0dd29c13..9e9d4dea 100644
--- a/source/popup/hooks/desktop.ts
+++ b/source/popup/hooks/desktop.ts
@@ -48,7 +48,7 @@ export function useDesktopConnectionState(): DesktopConnectionState {
     return DesktopConnectionState.Pending;
 }
 
-export function useEntriesForURL(url: string): Array<SearchResult> {
+export function useEntriesForURL(url: string | null): Array<SearchResult> {
     const performSearch = useCallback(async () => {
         if (!url) return [];
         const results = await searchEntriesByURL(url);
diff --git a/source/popup/queries/desktop.ts b/source/popup/queries/desktop.ts
index f573187a..ab40f5d4 100644
--- a/source/popup/queries/desktop.ts
+++ b/source/popup/queries/desktop.ts
@@ -19,7 +19,7 @@ export async function getDesktopConnectionAvailable(): Promise<boolean> {
     if (resp.error) {
         throw new Layerr(resp.error, "Failed checking desktop connection availability");
     }
-    return resp.available;
+    return resp.available ?? false;
 }
 
 export async function getOTPs(): Promise<Array<OTP>> {
@@ -29,7 +29,7 @@ export async function getOTPs(): Promise<Array<OTP>> {
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching OTPs from desktop application");
     }
-    return resp.otps;
+    return resp.otps ?? [];
 }
 
 export async function getRecentEntries(): Promise<Array<SearchResult>> {
@@ -40,7 +40,7 @@ export async function getRecentEntries(): Promise<Array<SearchResult>> {
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching recent entries from desktop application");
     }
-    return resp.searchResults;
+    return resp.searchResults ?? [];
 }
 
 export async function getVaultSources(): Promise<Array<VaultSourceDescription>> {
@@ -50,7 +50,7 @@ export async function getVaultSources(): Promise<Array<VaultSourceDescription>>
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching vaults from desktop application");
     }
-    return resp.vaultSources;
+    return resp.vaultSources ?? [];
 }
 
 export async function initiateDesktopConnectionRequest(): Promise<void> {
@@ -91,7 +91,7 @@ export async function searchEntriesByTerm(term: string): Promise<Array<SearchRes
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching search results from desktop application");
     }
-    return resp.searchResults;
+    return resp.searchResults ?? [];
 }
 
 export async function searchEntriesByURL(url: string): Promise<Array<SearchResult>> {
@@ -102,5 +102,5 @@ export async function searchEntriesByURL(url: string): Promise<Array<SearchResul
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching URL results from desktop application");
     }
-    return resp.searchResults;
+    return resp.searchResults ?? [];
 }
diff --git a/source/popup/services/entry.ts b/source/popup/services/entry.ts
index 29705ee9..c6e1d7ca 100644
--- a/source/popup/services/entry.ts
+++ b/source/popup/services/entry.ts
@@ -12,5 +12,5 @@ export async function openPageForEntry(item: SearchResult, autoLogin: boolean):
     if (resp.error) {
         throw new Layerr(resp.error, "Failed opening page");
     }
-    return resp.opened;
+    return resp.opened ?? false;
 }
diff --git a/source/shared/components/EntryResult.tsx b/source/shared/components/EntryResult.tsx
deleted file mode 100644
index e69de29b..00000000
diff --git a/source/shared/components/EntryResultList.tsx b/source/shared/components/EntryResultList.tsx
deleted file mode 100644
index ea18ed5f..00000000
--- a/source/shared/components/EntryResultList.tsx
+++ /dev/null
@@ -1,10 +0,0 @@
-import React from "react";
-import { SearchResult } from "buttercup";
-
-interface EntryResultListProps {
-    entries: Array<SearchResult>;
-}
-
-export function EntryResultList(props: EntryResultListProps) {
-
-}
diff --git a/source/shared/components/ErrorBoundary.tsx b/source/shared/components/ErrorBoundary.tsx
index 099f1be6..2d1fdcc8 100644
--- a/source/shared/components/ErrorBoundary.tsx
+++ b/source/shared/components/ErrorBoundary.tsx
@@ -26,7 +26,10 @@ export class ErrorBoundary extends Component {
         return { error };
     }
 
-    state = {
+    state: {
+        error: null | Error;
+        errorStack: string | null;
+    } = {
         error: null,
         errorStack: null
     };
diff --git a/source/shared/hooks/async.ts b/source/shared/hooks/async.ts
index 6e1a4f1d..53600c2f 100644
--- a/source/shared/hooks/async.ts
+++ b/source/shared/hooks/async.ts
@@ -17,13 +17,13 @@ export function useAsync<T extends any>(
     }: {
         clearOnExec?: boolean;
         updateInterval?: number | null;
-        valuesDiffer?: (existingValue: T, newValue: T) => boolean;
+        valuesDiffer?: (existingValue: T | null, newValue: T | null) => boolean;
     } = {}
 ): AsyncResult<T> {
     const mounted = useRef(false);
     const executing = useRef(false);
-    const [value, setValue] = useState<T>(null);
-    const [error, setError] = useState<Error>(null);
+    const [value, setValue] = useState<T | null>(null);
+    const [error, setError] = useState<Error | null>(null);
     const [loading, setLoading] = useState<boolean | null>(null);
     const [, setTimer] = useState<null | ReturnType<typeof setTimeout>>(null);
     const execute = useCallback(async () => {
@@ -58,7 +58,7 @@ export function useAsync<T extends any>(
     useEffect(() => {
         if (updateInterval === null) return;
         setTimer((existing) => {
-            clearTimeout(existing);
+            clearTimeout(existing as any);
             return null;
         });
         let newTimer: ReturnType<typeof setTimeout>;
@@ -100,9 +100,9 @@ export function useAsyncWithTimer<T extends any>(
     value: T | null;
 } {
     const mounted = useRef(false);
-    const allTimers = useRef([]);
+    const allTimers = useRef<Array<ReturnType<typeof setInterval>>>([]);
     const [time, setTime] = useState<number>(Date.now());
-    const [timer, setTimer] = useState<ReturnType<typeof setInterval>>(null);
+    const [timer, setTimer] = useState<ReturnType<typeof setInterval> | null>(null);
     const { error, loading, value } = useAsync(fn, [...deps, time]);
     const [lastValue, setLastValue] = useState(value);
     useEffect(() => {
@@ -117,7 +117,7 @@ export function useAsyncWithTimer<T extends any>(
     useEffect(() => {
         if (time === 0) return;
         if (error) {
-            clearInterval(timer);
+            clearInterval(timer as any);
             setTime(0);
             setTimer(null);
             return;
@@ -127,7 +127,7 @@ export function useAsyncWithTimer<T extends any>(
                 if (!mounted.current) return;
                 setTime(Date.now());
             }, delay);
-            allTimers.current.push(thisTimer);
+            allTimers.current.push(thisTimer as any);
             setTimer(thisTimer);
         }
     }, [time, timer, error, delay]);
diff --git a/source/shared/hooks/config.ts b/source/shared/hooks/config.ts
index 3c1ce7e8..06d137fa 100644
--- a/source/shared/hooks/config.ts
+++ b/source/shared/hooks/config.ts
@@ -14,7 +14,7 @@ export function useConfig(): [
     const { value, error } = useAsync(getConfig, [ts], {
         clearOnExec: false
     });
-    const [changeError, setChangeError] = useState<Error>(null);
+    const [changeError, setChangeError] = useState<Error | null>(null);
     const setConfigValue = useCallback(<T extends keyof Configuration>(setKey: T, value: Configuration[T]) => {
         setChangeError(null);
         setNewBackgroundValue(setKey, value)
diff --git a/source/shared/library/extension.ts b/source/shared/library/extension.ts
index 34c953e1..f587a1a8 100644
--- a/source/shared/library/extension.ts
+++ b/source/shared/library/extension.ts
@@ -2,12 +2,12 @@ import { getExtensionAPI } from "../extension.js";
 
 const NOOP = () => {};
 
-export async function createNewTab(url: string): Promise<chrome.tabs.Tab> {
+export async function createNewTab(url: string): Promise<chrome.tabs.Tab | null> {
     const browser = getExtensionAPI();
     if (!browser.tabs) {
         // Handle non-background scripts
         browser.runtime.sendMessage({ type: "open-tab", url });
-        return;
+        return null;
     }
     return new Promise<chrome.tabs.Tab>((resolve) => chrome.tabs.create({ url }, resolve));
 }
@@ -15,6 +15,7 @@ export async function createNewTab(url: string): Promise<chrome.tabs.Tab> {
 export function closeCurrentTab() {
     const browser = getExtensionAPI();
     browser.tabs.getCurrent((tab) => {
+        if (!tab?.id) return;
         browser.tabs.remove(tab.id, NOOP);
     });
 }
diff --git a/source/shared/library/version.ts b/source/shared/library/version.ts
index 3c772fda..bb427629 100644
--- a/source/shared/library/version.ts
+++ b/source/shared/library/version.ts
@@ -1,4 +1,4 @@
 // Do not edit this file - it is generated automatically at build time
 
-export const BUILD_DATE = "2024-04-06";
+export const BUILD_DATE = "2024-04-07";
 export const VERSION = "3.1.0";
diff --git a/source/shared/queries/config.ts b/source/shared/queries/config.ts
index 9c693b0c..519ab20b 100644
--- a/source/shared/queries/config.ts
+++ b/source/shared/queries/config.ts
@@ -9,6 +9,9 @@ export async function getConfig(): Promise<Configuration> {
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching application configuration");
     }
+    if (!resp.config) {
+        throw new Error("No config returned from background");
+    }
     return resp.config;
 }
 
@@ -24,5 +27,8 @@ export async function setConfigValue<T extends keyof Configuration>(
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching application configuration");
     }
+    if (!resp.config) {
+        throw new Error("No config returned from background");
+    }
     return resp.config;
 }
diff --git a/source/shared/types.ts b/source/shared/types.ts
index 6a5c2eb2..86d12936 100644
--- a/source/shared/types.ts
+++ b/source/shared/types.ts
@@ -78,7 +78,7 @@ export interface BackgroundResponse {
     available?: boolean;
     autoLogin?: SearchResult | null;
     config?: Configuration;
-    credentials?: Array<UsedCredentials>;
+    credentials?: Array<UsedCredentials | null>;
     domains?: Array<string>;
     entryID?: EntryID | null;
     error?: Error;
diff --git a/source/tab/library/dismount.ts b/source/tab/library/dismount.ts
index 35420755..cd3085b7 100644
--- a/source/tab/library/dismount.ts
+++ b/source/tab/library/dismount.ts
@@ -16,6 +16,9 @@ export function onElementDismount(el: HTMLElement, callback: () => void): void {
             callback();
         }
     });
+    if (!el.parentElement) {
+        throw new Error("No parent element found for target");
+    }
     mutObs.observe(el.parentElement, { childList: true });
     timer = setTimeout(() => {
         if (!el.parentElement) {
diff --git a/source/tab/services/LoginTracker.ts b/source/tab/services/LoginTracker.ts
index ed3a0ca5..82525f6f 100644
--- a/source/tab/services/LoginTracker.ts
+++ b/source/tab/services/LoginTracker.ts
@@ -17,7 +17,7 @@ interface LoginTrackerEvents {
     credentialsChanged: (event: { id: string; username: string; password: string; entry: boolean }) => void;
 }
 
-let __sharedTracker = null;
+let __sharedTracker: LoginTracker | null = null;
 
 export class LoginTracker extends EventEmitter<LoginTrackerEvents> {
     protected _connections: Array<Connection> = [];
@@ -32,9 +32,11 @@ export class LoginTracker extends EventEmitter<LoginTrackerEvents> {
         return this._url;
     }
 
-    getConnection(loginTarget: LoginTarget) {
-        return this._connections.find(
-            (conn) => conn.loginTarget === loginTarget || conn.loginTarget.form === loginTarget.form
+    getConnection(loginTarget: LoginTarget): Connection | null {
+        return (
+            this._connections.find(
+                (conn) => conn.loginTarget === loginTarget || conn.loginTarget.form === loginTarget.form
+            ) || null
         );
     }
 
diff --git a/source/tab/services/config.ts b/source/tab/services/config.ts
index 99b7b386..cde17a66 100644
--- a/source/tab/services/config.ts
+++ b/source/tab/services/config.ts
@@ -9,5 +9,8 @@ export async function getConfig(): Promise<Configuration> {
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching configuration");
     }
+    if (!resp.config) {
+        throw new Error("No config returned from background");
+    }
     return resp.config;
 }
diff --git a/source/tab/services/form.ts b/source/tab/services/form.ts
index f349e246..45f46671 100644
--- a/source/tab/services/form.ts
+++ b/source/tab/services/form.ts
@@ -11,6 +11,12 @@ import { FrameEvent, FrameEventType, TabEventType } from "../types.js";
 export function fillFormDetails(frameEvent: FrameEvent) {
     const { currentLoginTarget: loginTarget } = FORM;
     const { inputDetails } = frameEvent;
+    if (!inputDetails) {
+        throw new Error("No input details for form fill action");
+    }
+    if (!loginTarget) {
+        throw new Error("No login target found");
+    }
     if (inputDetails.username) {
         loginTarget.fillUsername(inputDetails.username);
     }
@@ -69,6 +75,18 @@ export async function initialise() {
                 throw new Error("Unexpected details input state");
             }
         } else if (tabEvent.type === TabEventType.OpenPopupDialog) {
+            if (!tabEvent.sourceURL) {
+                console.error("No source URL provided");
+                return;
+            }
+            if (!tabEvent.inputPosition) {
+                console.error("No input position provided");
+                return;
+            }
+            if (!tabEvent.inputType) {
+                console.error("No input type provided");
+                return;
+            }
             // Re-calculate based upon the iframe the message came from
             const frame = findIframeForWindow(tabEvent.sourceURL);
             if (!frame) {
@@ -78,7 +96,7 @@ export async function initialise() {
             const newPosition = recalculateRectForIframe(tabEvent.inputPosition, frame);
             // Show if top, or pass on to the next frame above
             if (FRAME.isTop) {
-                FORM.targetFormID = tabEvent.formID;
+                FORM.targetFormID = tabEvent.formID ?? null;
                 togglePopup(newPosition, tabEvent.inputType);
             } else {
                 sendTabEvent(
diff --git a/source/tab/services/formDetection.ts b/source/tab/services/formDetection.ts
index d43ba4e7..91ad3448 100644
--- a/source/tab/services/formDetection.ts
+++ b/source/tab/services/formDetection.ts
@@ -15,7 +15,7 @@ function filterLoginTarget(_: LoginTargetFeature, element: HTMLElement): boolean
 }
 
 function onIdentifiedTarget(callback: (target: LoginTarget) => void) {
-    const locatedForms = [];
+    const locatedForms: Array<HTMLElement> = [];
     const findTargets = () => {
         getLoginTargets(document, filterLoginTarget)
             .filter((target) => locatedForms.includes(target.form) === false)
diff --git a/source/tab/services/logins/disabled.ts b/source/tab/services/logins/disabled.ts
index e437c658..bec5fad8 100644
--- a/source/tab/services/logins/disabled.ts
+++ b/source/tab/services/logins/disabled.ts
@@ -9,5 +9,5 @@ export async function getDisabledDomains(): Promise<Array<string>> {
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching disabled login domains");
     }
-    return resp.domains;
+    return resp.domains ?? [];
 }
diff --git a/source/tab/services/logins/saving.ts b/source/tab/services/logins/saving.ts
index 934343cb..06976624 100644
--- a/source/tab/services/logins/saving.ts
+++ b/source/tab/services/logins/saving.ts
@@ -10,7 +10,7 @@ export async function getCredentialsForID(id: string): Promise<UsedCredentials |
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching saved credentials");
     }
-    return resp.credentials[0] ?? null;
+    return resp.credentials?.[0] ?? null;
 }
 
 export async function getLastSavedCredentials(): Promise<UsedCredentials | null> {
@@ -20,7 +20,7 @@ export async function getLastSavedCredentials(): Promise<UsedCredentials | null>
     if (resp.error) {
         throw new Layerr(resp.error, "Failed fetching last saved credentials");
     }
-    return resp.credentials[0] ?? null;
+    return resp.credentials?.[0] ?? null;
 }
 
 export function transferLoginCredentials(details: UsedCredentials) {
diff --git a/source/tab/services/logins/watcher.ts b/source/tab/services/logins/watcher.ts
index e268cdb9..a3566b89 100644
--- a/source/tab/services/logins/watcher.ts
+++ b/source/tab/services/logins/watcher.ts
@@ -48,16 +48,21 @@ export function watchCredentialsOnTarget(loginTarget: LoginTarget): void {
         loginTarget,
         (username, source) => {
             const connection = tracker.getConnection(loginTarget);
-            connection.entry = source === "fill";
-            connection.username = username;
+            if (connection) {
+                connection.entry = source === "fill";
+                connection.username = username;
+            }
         },
         (password, source) => {
             const connection = tracker.getConnection(loginTarget);
-            connection.entry = source === "fill";
-            connection.password = password;
+            if (connection) {
+                connection.entry = source === "fill";
+                connection.password = password;
+            }
         },
         () => {
             const connection = tracker.getConnection(loginTarget);
+            if (!connection) return;
             setTimeout(() => {
                 checkForLoginSaveAbility(connection.id);
             }, 300);
diff --git a/source/tab/services/messaging.ts b/source/tab/services/messaging.ts
index 87057d6a..07b3cbd1 100644
--- a/source/tab/services/messaging.ts
+++ b/source/tab/services/messaging.ts
@@ -46,7 +46,7 @@ export function listenForTabEvents(callback: (event: TabEvent) => void) {
         if (event.data?.type && Object.values(TabEventType).includes(event.data?.type)) {
             callback({
                 ...(event.data as TabEvent),
-                source: event.source
+                source: event.source ?? undefined
             });
         }
     });
diff --git a/source/tab/ui/launch.ts b/source/tab/ui/launch.ts
index 9636eeb0..42e486fc 100644
--- a/source/tab/ui/launch.ts
+++ b/source/tab/ui/launch.ts
@@ -128,6 +128,7 @@ function renderButtonStyle(input: HTMLInputElement, onClick: () => void, reattac
         // toggleInputDialog(input, DIALOG_TYPE_ENTRY_PICKER);
         onClick();
     };
+    // @ts-ignore
     mount(input.offsetParent, button);
     onElementDismount(button, () => {
         reattachCB();
diff --git a/source/tab/ui/popup.ts b/source/tab/ui/popup.ts
index b3a18766..22c0cc17 100644
--- a/source/tab/ui/popup.ts
+++ b/source/tab/ui/popup.ts
@@ -57,7 +57,7 @@ function buildNewPopup(inputRect: ElementRect, forInputType: InputType) {
         frame
     );
     mount(document.body, container);
-    const removeBodyResizeListener = onBodyResize(() => updatePopupPosition(__popup.inputRect));
+    const removeBodyResizeListener = onBodyResize(() => updatePopupPosition((__popup as LastPopup).inputRect));
     document.body.addEventListener("click", closePopup, false);
     __popup = {
         cleanup: () => {
diff --git a/tsconfig.json b/tsconfig.json
index 6a90ac4d..70ca63f2 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,14 +1,15 @@
 {
     "compilerOptions": {
-        "outDir": "./dist",
-        "module": "esnext",
-        "moduleResolution": "node",
-        "target": "ES6",
+        "allowSyntheticDefaultImports": true,
         "esModuleInterop": true,
-        "strict": false,
         "jsx": "react-jsx",
+        "module": "esnext",
+        "moduleResolution": "node",
+        "outDir": "./dist",
         "resolveJsonModule": true,
-        "allowSyntheticDefaultImports": true,
+        "strict": false,
+        "strictNullChecks": true,
+        "target": "ES6",
         "types": ["chrome", "react", "react-dom"]
     },
     "include": [