我如何处理未注册Firebase用户的扑朔迷离

Question

我希望未注册的用户单击登录按钮后无法访问下一页，但是我希望弹出窗口中显示一条消息。 我不知道该怎么办，请帮帮我！
我的登录代码：

class LoginManager {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  final GoogleSignIn googleSignIn = GoogleSignIn();

  Future<String> signInWithGoogle() async {
    final GoogleSignInAccount googleSignInAccount = await googleSignIn.signIn();
    final GoogleSignInAuthentication googleSignInAuthentication =
    await googleSignInAccount.authentication;

    final AuthCredential credential = GoogleAuthProvider.getCredential(
      accessToken: googleSignInAuthentication.accessToken,
      idToken: googleSignInAuthentication.idToken,
    );

    final AuthResult authResult = await _auth.signInWithCredential(credential);
    final FirebaseUser user = authResult.user;

      assert(!user.isAnonymous);
      assert(await user.getIdToken() != null);

      final FirebaseUser currentUser = await _auth.currentUser();
      assert(user.uid == currentUser.uid);
      return("Succesfully login");
  }
}

登录按钮的OnTap代码：

      onPressed: () {
                          if(sign.signInWithGoogle() == null){
                            showDialog(
                              context: context,
                              builder: (BuildContext context){
                                return AlertDialog(
                                  title: new Text("Alert Dialog title"),
                                  content: new Text("Alert Dialog body"),
                                  actions: <Widget>[
                                    // usually buttons at the bottom of the dialog
                                    new FlatButton(
                                      child: new Text("Close"),
                                      onPressed: () {
                                        Navigator.of(context).pop();
                                      },
                                    ),
                                  ],
                                );
                              },
                            );
                          } else {
                           sign.signInWithGoogle().whenComplete(() {
                              Navigator.pushReplacement(
                                  context,
                                  new MaterialPageRoute(
                                      builder: (
                                          BuildContext context) => new HomePage()));
                            });
                          }
                        },