diff --git a/OmiseSDK/Sources/OmiseSDK.swift b/OmiseSDK/Sources/OmiseSDK.swift
index 172db936..0ae67f15 100644
--- a/OmiseSDK/Sources/OmiseSDK.swift
+++ b/OmiseSDK/Sources/OmiseSDK.swift
@@ -6,7 +6,7 @@ public class OmiseSDK {
     public static var shared = OmiseSDK(publicKey: "pkey_")
 
     /// OmiseSDK version
-    public let version: String = "5.2.0"
+    public let version: String = "5.2.1"
 
     /// Public Key associated with this instance of OmiseSDK
     public let publicKey: String
@@ -219,7 +219,9 @@ public class OmiseSDK {
 
 private extension OmiseSDK {
     private func preloadCapabilityAPI() {
-        client.capability { _ in }
+        client.capability { _ in
+            // Preload capability and auto cache it as client.latestLoadedCapability
+        }
     }
 }
 
diff --git a/OmiseSDK/Sources/Views/Components/TextFieldView.swift b/OmiseSDK/Sources/Views/Components/TextFieldView.swift
index 89540798..bee01702 100644
--- a/OmiseSDK/Sources/Views/Components/TextFieldView.swift
+++ b/OmiseSDK/Sources/Views/Components/TextFieldView.swift
@@ -46,9 +46,9 @@ class TextFieldView: UIView {
     }()
 
     var onTextFieldShouldReturn: () -> (Bool) = { return false }
-    var onTextChanged: () -> Void = { }
-    var onBeginEditing: () -> Void = { }
-    var onEndEditing: () -> Void = { }
+    var onTextChanged: () -> Void = { /* Non-optional default empty implementation */ }
+    var onBeginEditing: () -> Void = { /* Non-optional default empty implementation */ }
+    var onEndEditing: () -> Void = { /* Non-optional default empty implementation */ }
 
     // swiftlint:disable attributes
     @ProxyProperty(\TextFieldView.textField.keyboardType) var keyboardType: UIKeyboardType
diff --git a/OmiseSDK/Sources/Views/Components/TextFields/OmiseTextField.swift b/OmiseSDK/Sources/Views/Components/TextFields/OmiseTextField.swift
index e730b489..33fca0b2 100644
--- a/OmiseSDK/Sources/Views/Components/TextFields/OmiseTextField.swift
+++ b/OmiseSDK/Sources/Views/Components/TextFields/OmiseTextField.swift
@@ -21,7 +21,7 @@ public class OmiseTextField: UITextField {
     }
 
     public var onTextFieldShouldReturn: () -> (Bool) = { return false }
-    public var onValueChanged: () -> Void = { }
+    public var onValueChanged: () -> Void = { /* Non-optional default empty implementation */ }
 
     @IBInspectable var borderWidth: CGFloat {
         get {
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/AtomePaymentFormViewModel.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/AtomePaymentFormViewModel.swift
index 649448ab..8cfb7e42 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/AtomePaymentFormViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/AtomePaymentFormViewModel.swift	
@@ -27,7 +27,7 @@ class AtomePaymentFormViewModel: AtomePaymentFormViewModelProtocol, CountryListV
         }
     }
 
-    var onSelectCountry: (Country) -> Void = { _ in }
+    var onSelectCountry: (Country) -> Void = { _ in /* Non-optional default empty implementation */ }
 
     var countryListViewModel: CountryListViewModelProtocol { return self }
     
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/Base/PaymentFormController.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/Base/PaymentFormController.swift
index c05f63a0..2e6ff0be 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/Base/PaymentFormController.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/Atome/Base/PaymentFormController.swift	
@@ -2,7 +2,7 @@ import Foundation
 import UIKit
 
 class PaymentFormController: UIViewController {
-    var onSubmitButtonTappedClosure: () -> Void = { }
+    var onSubmitButtonTappedClosure: () -> Void = { /* Non-optional default empty implementation */ }
 
     @ProxyProperty(\PaymentFormController.headerTextLabel.text)
     var details: String?
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/FPX/FPXPaymentFormController.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/FPX/FPXPaymentFormController.swift
index 7f69d5da..1311aa79 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/FPX/FPXPaymentFormController.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Other Payments/FPX/FPXPaymentFormController.swift	
@@ -133,7 +133,7 @@ class FPXPaymentFormController: UIViewController, PaymentFormUIController {
 
     @IBAction private func submitForm(_ sender: AnyObject) {
         emailValue = emailTextField.text?.trimmingCharacters(in: CharacterSet.whitespaces)
-        delegate?.fpxDidCompleteWith(email: emailValue) {}
+        delegate?.fpxDidCompleteWith(email: emailValue) { /* no action is required */ }
     }
 
     @IBAction private func validateFieldData(_ textField: OmiseTextField) {
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentController.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentController.swift
index e3793cdf..66b93079 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentController.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentController.swift	
@@ -4,7 +4,9 @@ class SelectPaymentController: UITableViewController {
 
     let viewModel: SelectPaymentPresentableProtocol
 
-    var customizeCellAtIndexPathClosure: (UITableViewCell, IndexPath) -> Void = { _, _ in }
+    var customizeCellAtIndexPathClosure: (UITableViewCell, IndexPath) -> Void = { _, _ in
+        // Non-optional default empty implementation
+    }
 
     init(viewModel: SelectPaymentPresentableProtocol) {
         self.viewModel = viewModel
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentPresentableProtocol.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentPresentableProtocol.swift
index fb19f2d2..51ab5736 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentPresentableProtocol.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/SelectPaymentPresentableProtocol.swift	
@@ -19,5 +19,5 @@ extension SelectPaymentPresentableProtocol {
     var errorMessage: String? { nil }
     var viewShowsCloseButton: Bool { false }
     var viewDisplayLargeTitle: Bool { false }
-    func viewDidTapClose() {}
+    func viewDidTapClose() { /* Default empty implementation */ }
 }
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectDuitNowOBWBankViewModel.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectDuitNowOBWBankViewModel.swift
index 1fc5c7ef..06955395 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectDuitNowOBWBankViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectDuitNowOBWBankViewModel.swift	
@@ -3,7 +3,7 @@ import UIKit
 class SelectDuitNowOBWBankViewModel {
     private weak var delegate: SelectSourcePaymentDelegate?
 
-    private var viewOnDataReloadHandler: () -> Void = { } {
+    private var viewOnDataReloadHandler: () -> Void = { /* Non-optional default empty implementation */ } {
         didSet {
             self.viewOnDataReloadHandler()
         }
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectFPXBankViewModel.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectFPXBankViewModel.swift
index 938766f3..bde82a62 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectFPXBankViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectFPXBankViewModel.swift	
@@ -6,7 +6,7 @@ class SelectFPXBankViewModel {
 
     let errorMessage: String?
 
-    private var viewOnDataReloadHandler: () -> Void = { } {
+    private var viewOnDataReloadHandler: () -> Void = { /* Non-optional default empty implementation */ } {
         didSet {
             self.viewOnDataReloadHandler()
         }
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectInstallmentTermsViewModel.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectInstallmentTermsViewModel.swift
index 156f2fe7..81ef32ed 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectInstallmentTermsViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectInstallmentTermsViewModel.swift	
@@ -4,7 +4,7 @@ class SelectInstallmentTermsViewModel {
     weak var delegate: SelectSourcePaymentDelegate?
     let sourceType: SourceType
 
-    private var viewOnDataReloadHandler: () -> Void = { } {
+    private var viewOnDataReloadHandler: () -> Void = { /* Non-optional default empty implementation */ } {
         didSet {
             self.viewOnDataReloadHandler()
         }
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectPaymentMethodViewModel.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectPaymentMethodViewModel.swift
index b8215e14..2a907d5f 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectPaymentMethodViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectPaymentMethodViewModel.swift	
@@ -12,7 +12,7 @@ class SelectPaymentMethodViewModel {
     private let filter: Filter
     private weak var delegate: SelectPaymentMethodDelegate?
 
-    private var viewOnDataReloadHandler: () -> Void = { } {
+    private var viewOnDataReloadHandler: () -> Void = { /* Non-optional default empty implementation */ } {
         didSet {
             self.viewOnDataReloadHandler()
         }
diff --git a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectSourceTypePaymentViewModel.swift b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectSourceTypePaymentViewModel.swift
index 4bf1f8a8..40d3e3fb 100644
--- a/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectSourceTypePaymentViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Choose Payment Methods/Select Payment From Lists/ViewModels/SelectSourceTypePaymentViewModel.swift	
@@ -4,7 +4,7 @@ class SelectSourceTypePaymentViewModel {
     private weak var delegate: SelectSourceTypeDelegate?
     private let title: String
 
-    private var viewOnDataReloadHandler: () -> Void = { } {
+    private var viewOnDataReloadHandler: () -> Void = { /* Non-optional default empty implementation */ } {
         didSet {
             self.viewOnDataReloadHandler()
         }
diff --git a/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CCVInfoController.swift b/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CCVInfoController.swift
index e2ffac4b..945b6c6f 100644
--- a/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CCVInfoController.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CCVInfoController.swift	
@@ -16,7 +16,7 @@ class CCVInfoController: UIViewController {
         }
     }
 
-    var onCloseTapped: () -> Void = { }
+    var onCloseTapped: () -> Void = { /* Non-optional default empty implementation */ }
 
     override func viewDidLoad() {
         super.viewDidLoad()
diff --git a/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CreditCardPaymentViewModel.swift b/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CreditCardPaymentViewModel.swift
index 78ab2a36..74951f52 100644
--- a/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CreditCardPaymentViewModel.swift	
+++ b/OmiseSDK/Sources/Views/Screens/Credit Card Payment/CreditCardPaymentViewModel.swift	
@@ -42,7 +42,8 @@ class CreditCardPaymentViewModel: CreditCardPaymentViewModelProtocol, CountryLis
             }
         }
     }
-    var onSelectCountry: (Country) -> Void = { _ in }
+    
+    var onSelectCountry: (Country) -> Void = { _ in /* Non-optional default empty implementation */ }
 
     func error(for field: AddressField, validate text: String?) -> String? {
         guard isAddressFieldsVisible else { return nil }