Navigator.pushReplacement gives Null check operator used on a null value. This time it is - state

Google signin provider class is not navigating to the next screen. Signin the user works, but the suer has to signin then leave the app and then re-open. After re-opening the app works fine with the user signed in.

Error log gives: [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value E/flutter ( 9884): #0 StatefulElement.state (package:flutter/src/widgets/framework.dart:4789:44) E/flutter ( 9884): #1 Navigator.of (package:flutter/src/widgets/navigator.dart:2730:47) E/flutter ( 9884): #2 GoogleSignInProvider.signInwithGoogle (package:blahblah/google_signin_provider.dart:37:17) E/flutter ( 9884): <asynchronous suspension>

The error points to a "state" in the Navigator class. How do I set it, so the app navigates?

class GoogleSignInProvider extends ChangeNotifier {

final FirebaseAuth _auth = FirebaseAuth.instance;
User user = firebaseAuth.currentUser;
final GoogleSignIn _googleSignIn = GoogleSignIn();

 Future<String> signInwithGoogle(BuildContext context) async {

  try {
    final GoogleSignInAccount googleSignInAccount =
    await _googleSignIn.signIn();
    final GoogleSignInAuthentication googleSignInAuthentication =
    await googleSignInAccount.authentication;
    final AuthCredential credential = GoogleAuthProvider.credential(
      accessToken: googleSignInAuthentication.accessToken,
      idToken: googleSignInAuthentication.idToken,
    );
    await _auth.signInWithCredential(credential);
  } on FirebaseAuthException catch (e) {
    print(e.message);
    throw e;
  }

  final User currentUser = FirebaseAuth.instance.currentUser;
  String uid = currentUser.uid;
  if (currentUser != null)
    await usersRef.doc(currentUser.uid).set({'id': uid, 'email': 
 currentUser.email,
      'username': currentUser.displayName, 'photoUrl': 
 currentUser.photoURL,
      'phone': '', 'dob': '', 'sex': ''});

 Navigator.of(context).pushReplacement(
      MaterialPageRoute(builder: (_) => TabScreen()));
  notifyListeners();
return null;
}

Future logout() async {
await _googleSignIn.disconnect();
FirebaseAuth.instance.signOut();
}
}

thanks for the help!