Unhandled Exception: Null check operator used on a null value on flutter FCM

Question

whenever the app start, it always give this warning. For now, it doesnt give any bug, but I'm afraid if it will turn into some bugs later on. This is the error

E/flutter (11083): [ERROR:flutter/lib/ui/ui_dart_state.cc(199)] Unhandled Exception: Null check operator used on a null value
E/flutter (11083): #0      MethodChannelFirebaseMessaging.registerBackgroundMessageHandler (package:firebase_messaging_platform_interface/src/method_channel/method_channel_messaging.dart:180:53)
E/flutter (11083): #1      FirebaseMessagingPlatform.onBackgroundMessage= (package:firebase_messaging_platform_interface/src/platform_interface/platform_interface_messaging.dart:102:16)
E/flutter (11083): #2      FirebaseMessaging.onBackgroundMessage (package:firebase_messaging/src/messaging.dart:85:31)
E/flutter (11083): #3      messageHandler (package:sobatku/main.dart:249:21)
E/flutter (11083): <asynchronous suspension>

And I already put the message handler outside of main function

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  messageHandler();
  runApp(App());
}

MessageHandler

Future<void> messageHandler() async {

  await Firebase.initializeApp();

  await FirebaseMessaging.instance
      .setForegroundNotificationPresentationOptions(
    alert: true,
    badge: true,
    sound: true,
  );

  FirebaseMessaging.onMessage.listen((RemoteMessage event) {
    LocalNotification.showNotification(event);
  });

  FirebaseMessaging.onBackgroundMessage((message) => _messageHandler(message));
}

The main.dart structure