I am not able to catch the data from firebase messaging when my app is closed in Flutter

I am trying to catch key-value data from firebase messaging, i.e. by receiving notifications from firebase. I made a class for functionality and then assigned values to two global variables. Still, when I receive notification and click on it, no data is shown in my ui. Even title and body of notification is not shown when I start my app from scratch, i.e. first I run my app, close it, receive notification and then open it by clicking on it. I want that whether my app is closed, running in background or opened, key-value data could be feteched and desplayed on UI.

Here is my code -

variables.dart -

import 'package:flutter/cupertino.dart';

ValueNotifier<String> data = ValueNotifier("");
ValueNotifier<String> bodyData = ValueNotifier("");

notification.dart -

import 'dart:async';

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_messaging/firebase_messaging.dart';
import 'package:flutter/material.dart';
import 'package:flutter_application_notifications/variables.dart';

Future<void> onBackgroundMessage(RemoteMessage message) async {
  await Firebase.initializeApp();

  if (message.data.containsKey("data")) {
    data.value = message.data["data"];
  }

  if (message.data.containsKey("body")) {
    bodyData.value = message.data['body'];
  }
}

class Messaging {
  final firebaseMessaging = FirebaseMessaging.instance;

  final streamController = StreamController<String>.broadcast();
  final titleController = StreamController<String>.broadcast();
  final bodyController = StreamController<String>.broadcast();

  setNotifications() {
    FirebaseMessaging.onBackgroundMessage(onBackgroundMessage);
    FirebaseMessaging.onMessage.listen((message) async {
      if (message.data.containsKey('data')) {
        streamController.sink.add(message.data['data']);
      }

      if (message.data.containsKey('notification')) {
        streamController.sink.add(message.data['notification']);
      }

      titleController.sink
          .add(message.notification?.title ?? "Recieved Null Title");
      bodyController.sink
          .add(message.notification?.body ?? "Recieved Null Body");
    });

    final token = firebaseMessaging
        .getToken()
        .then((value) => print("Token is - $value"));
  }

  dispose() {
    streamController.close();
    bodyController.close();
    titleController.close();
  }
}

main.dart -

import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';
import 'package:flutter_application_notifications/notification.dart';
import 'package:flutter_application_notifications/variables.dart';

void main() async {
  await init();
  runApp(const MyApp());
}

Future init() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);
  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String notificationTitle = "No title";
  String notificationBody = "No Body";
  String notificationData = "No Data";

  int _counter = 0;

  @override
  void initState() {
    super.initState();
    final firebaseMessaging = Messaging();
    firebaseMessaging.setNotifications();

    firebaseMessaging.streamController.stream.listen(_changeData);
    firebaseMessaging.bodyController.stream.listen(_changeBody);
    firebaseMessaging.titleController.stream.listen(_changeTitle);
  }

  _changeData(String message) {
    setState(() {
      notificationData = message;
    });
  }

  _changeTitle(String message) {
    setState(() {
      notificationTitle = message;
    });
  }

  _changeBody(String message) {
    setState(() {
      notificationBody = message;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              "Title is - $notificationTitle",
            ),
            Text(
              "Body is - $notificationBody",
            ),
            Text(
              "Data is - $notificationData",
            ),
            ValueListenableBuilder(
              valueListenable: data,
              builder: (context, value, child) {
                return Text('Received data is $value');
              },
            ),
          ],
        ),
      ),
    );
  }
}