我在flutter中使用firebase进行电话认证，我定义了一个名为verification id的后期变量，这样当otp被发送时，它会被初始化，我可以稍后使用它，但它总是空的，为什么呢？
这是我的腕尺：

import 'package:bloc/bloc.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:meta/meta.dart';

part 'phone_auth_state.dart';

class PhoneAuthCubit extends Cubit<PhoneAuthState> {
  late String verificationId;

  PhoneAuthCubit() : super(PhoneAuthInitial());

  Future<void> submitPhoneNumber(String phoneNumber) async {
    emit(PhoneAuthLoading());

    await FirebaseAuth.instance.verifyPhoneNumber(
      phoneNumber: '$phoneNumber',
      timeout: const Duration(seconds: 14),
      verificationCompleted: verificationCompleted,
      verificationFailed: verificationFailed,
      codeSent: codeSent,
      codeAutoRetrievalTimeout: codeAutoRetrievalTimeout,
    );
  }

  void verificationCompleted(PhoneAuthCredential credential) async {
    print('verificationCompleted');
    await signIn(credential);
  }

  void verificationFailed(FirebaseAuthException error) {
    print('verificationFailed : ${error.toString()}');
    emit(PhoneAuthFailure(errorMessage: error.toString()));
  }

  void codeSent(String verificationId, int? resendToken) {
    print('codeSent');
    this.verificationId = verificationId;
    emit(PhoneNumberSubmitted());
    print("khlst code sent");
  }

  void codeAutoRetrievalTimeout(String verificationId) {
    print('codeAutoRetrievalTimeout');
  }

  Future<void> submitOTP(String otpCode) async { 
    PhoneAuthCredential credential = PhoneAuthProvider.credential(
        verificationId: this.verificationId, smsCode: otpCode);

    await signIn(credential);
  }

  Future<void> signIn(PhoneAuthCredential credential) async {
    try {
      await FirebaseAuth.instance.signInWithCredential(credential);
      emit(PhoneOTPVerified());
    } catch (error) {
      emit(PhoneAuthFailure(errorMessage: error.toString()));
    }
  }

  Future<void> signOut() async {
    await FirebaseAuth.instance.signOut();
  }

  User getLoggedInUser() {
    User firebaseUser = FirebaseAuth.instance.currentUser!;
    return firebaseUser;
  }
}

字符串
在函数submitOTP中引发错误