Fix: black screen on startup

When user preferences did not exist, there was a chance to get a black screen on startup.

lib/src/app.dart

@@ -32,8 +32,10 @@ class _AppState extends State<App> with PreferencesMixin {
           appSettings.setActiveLocaleSilently(
               snapshot.data?.getString(prefCurrentLocale) ??
                   Platform.localeName);
-          appSettings.useDarkModeSilently =
-              snapshot.data!.getBool(prefThemeMode) as bool;
+          var pref = snapshot.data!.getBool(prefThemeMode);
+          if (pref != null) {
+            appSettings.useDarkModeSilently = pref;
+          }
           return Consumer<AppSettings>(
             builder: (context, appSettings, _) => MaterialApp(
               theme: ThemeData(primarySwatch: Colors.pink),