opt.: backup backward compatiblility

lib/data/model/app/backup.dart

@@ -18,7 +18,7 @@ const backupFormatVersion = 1;
 final _logger = Logger('Backup');
 
 @JsonSerializable()
-class Backup extends Mergeable {
+class Backup implements Mergeable {
   // backup format version
   final int version;
   final String date;
@@ -28,9 +28,9 @@ class Backup extends Mergeable {
   final Map<String, dynamic> container;
   final Map<String, dynamic> history;
   final int? lastModTime;
-  final Map<String, dynamic> settings;
+  final Map<String, dynamic>? settings;
 
-  Backup({
+  const Backup({
     required this.version,
     required this.date,
     required this.spis,
@@ -181,22 +181,25 @@ class Backup extends Mergeable {
     }
 
     // Settings
-    if (force) {
-      Stores.setting.box.putAll(settings);
-    } else {
-      final nowSettings = Stores.setting.box.keys.toSet();
-      final bakSettings = settings.keys.toSet();
-      final newSettings = bakSettings.difference(nowSettings);
-      final delSettings = nowSettings.difference(bakSettings);
-      final updateSettings = nowSettings.intersection(bakSettings);
-      for (final s in newSettings) {
-        Stores.setting.box.put(s, settings[s]);
-      }
-      for (final s in delSettings) {
-        Stores.setting.box.delete(s);
-      }
-      for (final s in updateSettings) {
-        Stores.setting.box.put(s, settings[s]);
+    final settings_ = settings;
+    if (settings_ != null) {
+      if (force) {
+        Stores.setting.box.putAll(settings_);
+      } else {
+        final nowSettings = Stores.setting.box.keys.toSet();
+        final bakSettings = settings_.keys.toSet();
+        final newSettings = bakSettings.difference(nowSettings);
+        final delSettings = nowSettings.difference(bakSettings);
+        final updateSettings = nowSettings.intersection(bakSettings);
+        for (final s in newSettings) {
+          Stores.setting.box.put(s, settings_[s]);
+        }
+        for (final s in delSettings) {
+          Stores.setting.box.delete(s);
+        }
+        for (final s in updateSettings) {
+          Stores.setting.box.put(s, settings_[s]);
+        }
       }
     }