docs(firebase_messaging): Update example app & documentation for initializing Firebase app from top-level.

docs/messaging/usage.mdx

@@ -221,16 +221,23 @@ There are a few things to keep in mind about your background message handler:
 2. It must be a top-level function (e.g. not a class method which requires initialization).
 
 ```dart
+// We recommend initializing the app from the top-level so it can be shared with your background message handler
+// See this issue for further details: https://github.com/FirebaseExtended/flutterfire/issues/6087
+final initApp = Firebase.initializeApp();
+
 Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
   // If you're going to use other Firebase services in the background, such as Firestore,
-  // make sure you call `initializeApp` before using other Firebase services.
-  await Firebase.initializeApp();
+  // make sure you await `initApp` before using other Firebase services.
+  await initApp;
 
   print("Handling a background message: ${message.messageId}");
 }
 
-void main() {
+void main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+  await initApp;
   FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);
+
   runApp(MyApp());
 }
 ```

packages/firebase_messaging/firebase_messaging/example/lib/main.dart

@@ -18,14 +18,19 @@ import 'message_list.dart';
 import 'permissions.dart';
 import 'token_monitor.dart';
 
+/// In light of https://github.com/FirebaseExtended/flutterfire/issues/6087,
+/// we make the initialized Firebase app top-level so it can be shared with the background message handler & the app.
+final initApp =
+    Firebase.initializeApp(options: DefaultFirebaseConfig.platformOptions);
+
 /// Define a top-level named handler which background/terminated messages will
 /// call.
 ///
 /// To verify things are working, check out the native platform logs.
 Future<void> _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
   // If you're going to use other Firebase services in the background, such as Firestore,
   // make sure you call `initializeApp` before using other Firebase services.
-  await Firebase.initializeApp(options: DefaultFirebaseConfig.platformOptions);
+  await initApp;
   print('Handling a background message ${message.messageId}');
 }
 
@@ -37,14 +42,7 @@ late FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin;
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
-  await Firebase.initializeApp(
-    options: const FirebaseOptions(
-      apiKey: 'AIzaSyAHAsf51D0A407EklG1bs-5wA7EbyfNFg0',
-      appId: '1:448618578101:ios:0b11ed8263232715ac3efc',
-      messagingSenderId: '448618578101',
-      projectId: 'react-native-firebase-testing',
-    ),
-  );
+  await initApp;
 
   // Set the background messaging handler early on, as a named top-level function
   FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);