"No Firebase App '[DEFAULT]' has been created – call Firebase.initializeApp() in Flutter and Firebase" Error shown but has been initiated already

Notice a few questions here regarding this topic, tried the solutions but still no fix to the problem in my code. I am making a sign up screen for my app in Android Studio, but when I press submit signup form, Flutter gives me this error, although I have initiated Firebase.initializeApp() already in my main.dart file. Why is this showing and how do I fix this?

In my Main.dart file

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await SystemChrome.setPreferredOrientations([
    DeviceOrientation.portraitUp,
  ]); // This makes the phone locked in portrait mode, preventing landscape usage
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  DarkThemeProvider themeChangeProvider = DarkThemeProvider();

  void getCurrentAppTheme() async {
    themeChangeProvider.setDarkTheme = await themeChangeProvider.darkThemePrefs
        .getTheme(); //calling dark theme provider to call dark theme prefs method
  } // fetching theme from shared preferences, if user chose value, last value chosen will be fetched and applied

  @override
  void initState() {
    getCurrentAppTheme();
    super.initState();
    Firebase.initializeApp().whenComplete(() {
      print("Intialised");
      setState(() {});
    });
  }

  final Future<FirebaseApp> _firebaseInitialisation = Firebase.initializeApp();

  @override
  Widget build(BuildContext context) {

    return FutureBuilder(
        future: _firebaseInitialisation,
        builder: (context, snapshot) {
          if (snapshot.connectionState == ConnectionState.waiting) {
            return const MaterialApp(

When pressing the button to register in a separate file, the code is

bool _isLoading = false; 
  void _submitFormOnRegister() async {
    final isValid = _formKey.currentState!.validate();
    FocusScope.of(context).unfocus();
    setState(() {
      _isLoading = true;
    });
    if (isValid) {
      _formKey.currentState!.save();
      try { //useful to catch any error
        await authInstance.createUserWithEmailAndPassword( //Look below for what authInstance is
            email: _emailTextController.text.toLowerCase().trim(),
            password: _passwordTextController.text.trim());
        print('Registered');
      } on FirebaseException catch (error) {
        UniversalFunctions.errorDialog(subtitle: '${error.message})', context: context);
        setState(() {
          _isLoading = false;
        });
      } catch (error) {
        UniversalFunctions.errorDialog(subtitle: '$error', context: context);
        setState(() {
          _isLoading = false;
        });
      } finally {
        setState(() {
          _isLoading = false;
        });
      }
    }
  }

the authInstance is in a separate file:

import 'package:firebase_auth/firebase_auth.dart';

final FirebaseAuth authInstance  = FirebaseAuth.instance;
final User? user = authInstance.currentUser;
final uid = user!.uid;

I suspected the button was activating before the firebase.initialise was being called but I’m not sure as I stated it at the front in the main. I saw a solution to add it to the

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

But it does not work (due to the other code having it locking screen format) and it doesn’t work when I remove that.