"No Firebase App '[DEFAULT]' has been created" despite "initializeApp()" has been called (Flutter, Firebase) - flutter

Question

I have the following workpiece of a starting widget:
import 'package:firebase_auth/firebase_auth.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:flutter/material.dart';
import 'firebase_options.dart';
import 'apps/auth_app.dart';
import 'apps/main_app.dart';
class StartingWidget extends StatelessWidget {
const StartingWidget({super.key});
void _initFirebase() async {
//WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
}
void _addAuthStatusListener() {
try {
FirebaseAuth.instance.authStateChanges().listen((User? user) {
if (user != null) {
//runApp(const MainApp());
} else {
//runApp(const AuthApp());
}
});
} catch (e) {
print(e.toString());
}
}
#override
Widget build(BuildContext context) {
_initFirebase();
_addAuthStatusListener();
return const Scaffold(
body: CircularProgressIndicator(),
);
}
}
When I start it on an Android emulator, I get the "No Firebase App '[DEFAULT]' has been created" error at line
FirebaseAuth.instance.authStateChanges().listen((User? user) {
despite Firebase.initializeApp() was called before. Uncommenting
WidgetsFlutterBinding.ensureInitialized();
doesn't change anything.

Answer 1

The reason is simple:
You have not waited for your _initFirebase() to complete, before you call _addAuthStatusListener(), which uses the Firebase app!
You also can't wait inside your build method (it has to render immediately and therefore can't be async), so I suggest you call _initFirebase() from inside _addAuthStatusListener() instead:
#override
Widget build(BuildContext context) {
// _initFirebase(); // <- Remove this here!
_addAuthStatusListener();
return const Scaffold(
body: CircularProgressIndicator(),
);
}
...
void _addAuthStatusListener() async { // Make async!
await _initFirebase(); // <- Add _initFirebase() here, with "await"!
try {
FirebaseAuth.instance.authStateChanges().listen((User? user) {
if (user != null) {
//runApp(const MainApp());
} else {
//runApp(const AuthApp());
}
});
} catch (e) {
print(e.toString());
}
}
That should do it! 🙂
Explanation
What you have to understand about async functions is that calling them without the await keyword only STARTS them. Then, the reader continues, without waiting for them to finish. Meanwhile, the async function also continues with its own stuff, in parallell with the main reader, until it's done.
And so, in your case above, you started the _initFirebase(), and inside there, a reader sat down and waited for Firebase.initializeApp() to complete. But while that was going on, the main reader continued to the next line in the build() method, which was _addAuthStatusListener(). And this function had plenty of time to make it to where it would have needed your completely initialized Firebase app (the line where you got the error), before said initialization had a chance to complete.
Later on, the Firebase.initializeApp() command would have completed, soon after which the entire _initFirebase() function would have completed, but by then, your app had already crashed.
Adding "await" before _initFirebase() makes sure, however, that the reader can't continue to the next line until this function has completed, even though it is async.