无法从sqflite数据库中获取所有用户并显示在列表中

dependencies:
  sqflite: ^x.x.x

// 导入必要的库
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';

// 定义用户模型类
class User {
  final int id;
  final String name;
  final int age;

  User({this.id, this.name, this.age});
}

// 创建数据库帮助类
class DatabaseHelper {
  static final DatabaseHelper _instance = DatabaseHelper.internal();
  factory DatabaseHelper() => _instance;

  static Database _db;

  Future<Database> get db async {
    if (_db != null) {
      return _db;
    }
    _db = await initDb();
    return _db;
  }

  DatabaseHelper.internal();

  initDb() async {
    String databasesPath = await getDatabasesPath();
    String path = join(databasesPath, 'mydatabase.db');

    var db = await openDatabase(path, version: 1, onCreate: _onCreate);
    return db;
  }

  void _onCreate(Database db, int version) async {
    await db.execute(
        'CREATE TABLE Users(id INTEGER PRIMARY KEY, name TEXT, age INTEGER)');
  }

  Future<List<User>> getAllUsers() async {
    var dbClient = await db;
    List<Map> list = await dbClient.rawQuery('SELECT * FROM Users');
    List<User> users = [];
    for (var i = 0; i < list.length; i++) {
      users.add(User(
        id: list[i]['id'],
        name: list[i]['name'],
        age: list[i]['age'],
      ));
    }
    return users;
  }
}

// 创建数据访问对象（DAO）类
class UserDao {
  final dbHelper = DatabaseHelper();

  Future<List<User>> getAllUsers() {
    return dbHelper.getAllUsers();
  }
}

// 在界面中使用数据访问对象（DAO）类获取所有用户并显示在列表中
class UserListPage extends StatefulWidget {
  @override
  _UserListPageState createState() => _UserListPageState();
}

class _UserListPageState extends State<UserListPage> {
  final userDao = UserDao();
  List<User> users = [];

  @override
  void initState() {
    super.initState();
    _loadUsers();
  }

  _loadUsers() async {
    List<User> userList = await userDao.getAllUsers();
    setState(() {
      users = userList;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('User List'),
      ),
      body: ListView.builder(
        itemCount: users.length,
        itemBuilder: (context, index) {
          return ListTile(
            title: Text(users[index].name),
            subtitle: Text('Age: ${users[index].age}'),
          );
        },
      ),
    );
  }
}