Flutter showdialog or page when app in background FCM

i've implemented a function to handle message received onbackground from FCM. Now i need to show something to user when the app is in background.

I've imported get_it injectable stacked_services

and tried to create a customdialog that show up without passing a context. Done when the app is open but nothing when app is in background.

in main Showdialog is initialized:

  await setupLocator();
  setupDialogUi();
  PushNotificationsManager().init();

and a static dialogService too:

variables.dialogService = locator<DialogService>();

navigatorkey setted to

 navigatorKey: StackedService.navigatorKey,

then, in FCM pushnotificationmanager i have:

Future<dynamic> myBackgroundMessageHandler(Map<String, dynamic> message) async {

  print('on background $message');

  variables.dialogService.showCustomDialog(
    variant: DialogType.basic,  
    title: 'This is a custom UI with Text as main button',
    description: 'Sheck out the builder in the dialog_ui_register.dart file',
    mainButtonTitle: 'Ok',
  );

}
class PushNotificationsManager {

  PushNotificationsManager._();

  factory PushNotificationsManager() => _instance;

  static final PushNotificationsManager _instance = PushNotificationsManager._();

  final FirebaseMessaging _firebaseMessaging = FirebaseMessaging();
  bool _initialized = false;



  Future<void> init() async {
    if (!_initialized) {
      // For iOS request permission first.
      _firebaseMessaging.requestNotificationPermissions();
      _firebaseMessaging.configure(
        onMessage: (Map<String, dynamic> message) async {
          // showNotificationWithoutSound();
          print('firebase ########test ########## on message $message');

          variables.dialogService.showCustomDialog(
            variant: DialogType.basic,
            title: 'This is a custom UI with Text as main button',
            description: 'Sheck out the builder in the dialog_ui_register.dart file',
            mainButtonTitle: 'Ok',
          );
          // }
        },
        onBackgroundMessage: myBackgroundMessageHandler,
        onResume: (Map<String, dynamic> message) async {
          print('firebase ########test ########## on resume $message');
        },
        onLaunch: (Map<String, dynamic> message) async {
          print('firebase ########test ########## on launch $message');
        },


      );

      // For testing purposes print the Firebase Messaging token
      String token = await _firebaseMessaging.getToken();
      print("FirebaseMessaging token: $token");

      _initialized = true;
    }
  }
}

onMessage show correctly the dialog but myBackgroundMessageHandler throw an error:

NoSuchMethodError: The method 'showCustomDialog' was called on null.

Any suggestion to show a dialog or a widget or something when the app is in background?