我有以下代码：

我使用云Firestore作为数据库

用户模型：

class DbUser {
  String id;
  final String authUserID;
  final String userName;
  final List<String>? itemsForSale;
  final List<String>? itemFavourites;
  final List<String>? bids;

  DbUser(
      {this.id = '',
      required this.authUserID,
      required this.userName,
      this.itemsForSale,
      this.itemFavourites,
      this.bids});

  Map<String, dynamic> toJson() => {
        'id': id,
        'authUserID': authUserID,
        'userName': userName,
        'itemsForSale': itemsForSale,
        'itemFavourites': itemFavourites,
        'bids': bids,
      };

  static DbUser fromJson(Map<String, dynamic> json) => DbUser(
        id: json['id'],
        authUserID: json['authUserID'],
        userName: json['userName'],
        itemsForSale: json['itemsForSale'] is Iterable
            ? List.from(json['itemsForSale'])
            : null,
        itemFavourites: json['itemFavourites'] is Iterable
            ? List.from(json['itemFavourites'])
            : null,
        bids: json['bids'] is Iterable ? List.from(json['bids']) : null,
      );
}

存储库类

final _firestoreDB = FirebaseFirestore.instance;


  Future<DbUser?> getDBUserByDBUserId({required String dbUserID}) async {
    final docUser = _firestoreDB.collection('users').doc(dbUserID);
    final snapshot = await docUser.get();

    if (snapshot.exists) {
      return DbUser.fromJson(snapshot.data()!);
    }

    return null;
  }

snapshot.exists返回false。我不明白为什么？我的snapshot返回null，但我不明白为什么它会这样做，有人能帮帮我吗？
谢谢