无法返回从flutter中的firestore数据创建的用户对象的自定义' user‘实例

import 'package:cloud_firestore/cloud_firestore.dart';

class User {
  final String id;
  final String name;
  final String email;

  User({required this.id, required this.name, required this.email});

  // 从Map创建User实例的工厂构造函数
  factory User.fromDocumentSnapshot(DocumentSnapshot snapshot) {
    return User(
      id: snapshot.id,
      name: snapshot['name'],
      email: snapshot['email'],
    );
  }
}

void fetchUser() async {
  // 假设你已经设置了Firestore实例
  CollectionReference usersCollection = FirebaseFirestore.instance.collection('users');

  // 获取特定用户的文档引用
  DocumentReference userRef = usersCollection.doc('userId');

  // 获取文档快照
  DocumentSnapshot userSnapshot = await userRef.get();

  if (userSnapshot.exists()) {
    // 从文档快照创建User实例
    User user = User.fromDocumentSnapshot(userSnapshot);
    print('User: $user');
  } else {
    print('User does not exist');
  }
}