Flutter - How to link social account using Firebase facebook and google plugin - flutter

Question

I have a flutter app and would like the user to authenticate both with FB and Google. I do not want multiple accounts, just a single account that links both.
I am using :
firebase_auth 0.15.1
google_sign_in 4.0.14
facebook_plugin 3.0.0
I am not able to get the email address of the user when the user's account already exist with a different provider. The email is needed in order to get the list of providers for that user using the API call "fetchSignInMethodsForEmail"
Here is an example:
1: User login with Google credentials. The account is created in firebase and google is linked.
2: The user now logoff
3: The user now tries to login with FB with the same email.
-- User get the following error
code:"ERROR_ACCOUNT_EXISTS_WITH_DIFFERENT_CREDENTIAL"
details: null
message: "An account already exists with the same email address but different sign-in credentials. Sign in using a provider associated with this email address., **null**))
As you can see the email is null. I need the email in order to get a list of providers. Then I can redirect the user to correct provider
here is a snippet of my code
Future<FirebaseUser> signInWithFacebook({bool isIos}) async {
AuthCredential credential;
try {
final FacebookLoginResult result = await facebookLogin.logIn(['email']);
if (result.accessToken != null) {
credential = FacebookAuthProvider.getCredential(
accessToken: result.accessToken.token
);
AuthResult authResult = await _firebaseAuth.signInWithCredential(
credential,
);
firebaseUser = authResult.user;
return firebaseUser;
} catch (e) {
throw Exception(e);
}
return null;
}
Thanks for your help