Flutter SQLite数据库，在尝试将数据库中的所有行显示为列表视图时，调用了getter 'length‘on null错误

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

// 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();

  // 初始化数据库
  initDb() async {
    String databasesPath = await getDatabasesPath();
    String path = join(databasesPath, 'mydatabase.db');

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

  // 创建表
  void _onCreate(Database db, int version) async {
    await db.execute(
        'CREATE TABLE IF NOT EXISTS mytable (id INTEGER PRIMARY KEY, name TEXT)');
  }

  // 查询所有行
  Future<List<Map<String, dynamic>>?> getAllRows() async {
    var dbClient = await db;
    var result = await dbClient?.query('mytable');
    return result;
  }
}

// 创建列表视图
class MyListView extends StatefulWidget {
  @override
  _MyListViewState createState() => _MyListViewState();
}

class _MyListViewState extends State<MyListView> {
  List<Map<String, dynamic>>? rows = [];

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

  // 获取数据库中的所有行
  fetchRows() async {
    var dbHelper = DatabaseHelper();
    var result = await dbHelper.getAllRows();
    setState(() {
      rows = result;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (rows == null) {
      return Center(
        child: CircularProgressIndicator(),
      );
    } else if (rows!.isEmpty) {
      return Center(
        child: Text('数据库中没有数据'),
      );
    } else {
      return ListView.builder(
        itemCount: rows!.length,
        itemBuilder: (context, index) {
          return ListTile(
            title: Text(rows![index]['name']),
          );
        },
      );
    }
  }
}

// 在页面中使用列表视图
class MyPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('数据库列表'),
      ),
      body: MyListView(),
    );
  }
}

// 运行页面
void main() {
  runApp(MaterialApp(
    home: MyPage(),
  ));
}