Fix issue on first start

lib/helpers/preferences_helper.dart

@@ -74,8 +74,7 @@ class PreferencesHelper {
 
   /// Get all settings as an [ApplicationPreferences] object
   ApplicationPreferences get preferences => ApplicationPreferences(
-        enableDarkMode:
-            getBool(PreferencesKeyList.ENABLE_DARK_THEME, alternative: null),
+        enableDarkMode: getBool(PreferencesKeyList.ENABLE_DARK_THEME),
       );
 }
 

lib/models/application_preferences.dart

@@ -9,5 +9,5 @@ class ApplicationPreferences {
 
   const ApplicationPreferences({
     @required this.enableDarkMode,
-  });
+  }) : assert(enableDarkMode != null);
 }