Added dialog to ask users to enable Notifications

lib/app.dart

@@ -53,6 +53,7 @@ class _AppState extends State<App> {
     }
 
     FirebaseMessaging.onMessage.listen((message) => NotificationController.onForegroundMessageHandler(message, context));
+    FirebaseMessaging.onMessageOpenedApp.listen((message) => NotificationController.onForegroundMessageHandler(message, context));
     FirebaseMessaging.onBackgroundMessage(NotificationController.onBackgroundMessageHandler);
 
     super.initState();