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I have a Flutter web based game wherein I am trying to convert anonymous user to google registered user in Firebase. I recently upgraded Flutter from v2.3 to v3.3.8 and corresponding Firebase packages too. Here are the upgraded versions of Firebase packages

firebase_core: ^2.1.1 firebase_auth: ^4.1.1 google_sign_in: ^5.4.1 firebase_database: ^10.0.4 firebase_analytics: ^10.0.4

By default when the user hits the web page he is signed in anonymously into Firebase. When the user clicks on the Sign in With Google button, I call the below function:

Future<bool> signInWithGoogle({required BuildContext context}) async {


UserCredential? gCredential;

if (kIsWeb) {
  GoogleAuthProvider authProvider = GoogleAuthProvider();

  try {


    debugPrint('Inside sign in with Google');
    gCredential = await anonUser?.linkWithPopup(authProvider);
    debugPrint('After linkWithPopup');
    guser = gCredential?.user;
    FirebaseAuth.instance.setPersistence(Persistence
        .LOCAL); // in order to maintain session when user closes browser and opens again

    if (guser != null) {
      email = guser?.email ?? '';
      _token = await guser?.getIdToken();
      gname = guser?.displayName ?? '';
      guid = guser?.uid ?? '';
      gimage = guser?.photoURL ?? '';
      _userId = guid;
      googleSignIn = true;
    }

    debugPrint('***Google user is $guser and name is $gname');
  } on FirebaseAuthException catch (error) {
    switch (error.code) {
      case "provider-already-linked":
        debugPrint("The provider has already been linked to the user.");
        break;
      case "invalid-credential":
        debugPrint("The provider's credential is not valid.");
        break;
      case "credential-already-in-use":
        {
          debugPrint('In credential in use ${error.credential}');

          if (error.credential != null) {
            gCredential = await FirebaseAuth.instance
                .signInWithCredential(error.credential!);
            debugPrint('After sign in with credential');

            guser = gCredential.user;
          }
          FirebaseAuth.instance.setPersistence(Persistence
              .LOCAL); // in order to maintain session when user closes browser and opens again

          if (guser != null) {
            await guser?.reload(); // reloading to fix firebase bug

            guser = FirebaseAuth.instance.currentUser;
            email = guser?.email ?? '';
            _token = await guser?.getIdToken();
            gname = guser?.displayName ?? '';
            guid = guser?.uid ?? '';
            gimage = guser?.photoURL ?? '';
            _userId = guid;
            googleSignIn = true;
            await readStats();
          }

          debugPrint(
              "The account corresponding to the credential already exists, "
              "or is already linked to a Firebase User $guser and ${anonUser?.uid}. ${error.email} ${error.credential} ${guser?.displayName}");
        }
    }
  }

When I sign in, I am using an email that has already been used before so it is associated with some previous anonymous user in Firebase. Hence it goes into the credential-already-in-use exception. Here are the logs - you can see error.credential is coming null.

Inside sign in with Google
In credential in use null
The account corresponding to the credential already exists, or is already linked to a Firebase User null and N6ANc27jqGbFfp7zgPX9WmLktpp2. null null null

This same code was working perfectly fine before I upgraded. My earlier version of firebase packages was this:

firebase_core: 1.12.0 firebase_auth: ^3.3.6

With those packages, I was getting the error.credential and I could switch the user by signing in with the new credential.

This is the output of my flutter doctor -v:

[√] Flutter (Channel stable, 3.3.8, on Microsoft Windows [Version 10.0.19045.2251], locale en-IN)
    • Flutter version 3.3.8 on channel stable at D:\flutter
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 52b3dc25f6 (5 days ago), 2022-11-09 12:09:26 +0800
    • Engine revision 857bd6b74c
    • Dart version 2.18.4
    • DevTools version 2.15.0

[√] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    • Android SDK at C:\Users\Radha\AppData\Local\Android\sdk
    • Platform android-31, build-tools 30.0.3
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)
    • All Android licenses accepted.

[√] Chrome - develop for the web
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[√] Visual Studio - develop for Windows (Visual Studio Community 2022 17.0.6)
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.0.32126.317
    • Windows 10 SDK version 10.0.19041.0

[√] Android Studio (version 4.1)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin can be installed from:
       https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
       https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01)

[√] VS Code (version 1.73.1)
    • VS Code at C:\Users\Radha\AppData\Local\Programs\Microsoft VS Code
    • Flutter extension version 3.24.0

[√] Connected device (3 available)
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.19045.2251]
    • Chrome (web)      • chrome  • web-javascript • Google Chrome 107.0.5304.107
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 107.0.1418.42

[√] HTTP Host Availability
    • All required HTTP hosts are available

• No issues found!

Appreciate if anyone can share pointers to help with this issue.

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