Flutter how to listen for email verification before user can sign in - firebase

My app allows the user to register and sends an email for verification. However i have a problem where the user can sign in with the new account whether the email is verified or not. I have looked and cannot find a solution to this bearing in mind i'm new to flutter

My Auth Code

  Future<String> signIn(String email, String password) async {
    AuthResult result = await _firebaseAuth.signInWithEmailAndPassword(
        email: email, password: password);
    FirebaseUser user = result.user;
    if (user.isEmailVerified) {
      return user.uid;
    } else {
      return null;
    }
  }

  Future<String> signUp(String email, String password) async {
    AuthResult result = await _firebaseAuth.createUserWithEmailAndPassword(
        email: email, password: password);
    FirebaseUser user = result.user;
    try {
      await user.sendEmailVerification();
      return user.uid;
    } catch (e) {
      print("An error occurred while trying to send email verification");
      print(e.message);
    }
  }

sign in method

try {
        if (_isLoginForm) {
          userId = await widget.auth.signIn(_email, _password);
          print('Signed in: $userId');
        } else {
          userId = await widget.auth.signUp(_email, _password);
          //widget.auth.sendEmailVerification();
          _showVerifyEmailSentDialog();
          print('Signed up user: $userId');
        }
        setState(() {
          _isLoading = false;
        });

        if (userId.length > 0 && userId != null && _isLoginForm) {
          widget.loginCallback();
        }