Current User is null while isSignedIn Method returns true google_sign_in Flutter

I am using google sign in api in Flutter for Logging In. isSignedIn method returns true but the current user is getting null.

_googleSignIn.isSignedIn()
    .then((value){
        if(value){ // returns true in case of already logged In
          Future.delayed(Duration.zero, () async {
            _currentUser = _googleSignIn.currentUser;
            var googleKey = await _currentUser?.authentication;
            accessToken = googleKey?.accessToken;
            print("Access Token - $accessToken");
            print(_currentUser);
          });

        }});