未处理的异常：空值检查运算符用于空值 E/flutter (28057)：#0 DBHelper._createDB

Question

我为数据库操作创建了一个class，当我触发插入数据库打印的功能时出现这个错误：未处理的异常：空检查运算符用于空值 E/flutter (28057): #0 DBHelper._createDB，一旦初始化 DBHelper 的对象，也不会出现打印。我认为 null-able _db 对象是原因。解决方案是什么？

数据库class:

class DBHelper {
  DBHelper._();
  factory DBHelper() => instance;
  static final DBHelper instance = DBHelper._();
  Database? _db;

  Future<Database> _createDB() async {
    // If the app is opened just now
    if (_db == null) return _db!;
    // Open db too create a db
    // or to take an instance of it

    // Get the default databases location.
    // and join the name of app db
    String path = join(await getDatabasesPath(), 'todo.db');
    return _db = await openDatabase(path, version: 1,
        onCreate: (Database database, int version) {

      database.execute(
          "CREATE TABLE tasks(id integer autoincrement PRIMARY KEY,content varchar(150),status varchar(9),isFinished BOOLEAN) ");
      print("\n TABLE executed \n");
    }, onOpen: (_) {
      print("\n opened \n ");
    });
  }

  Future<int> insertNoteToDB(NoteDatabaseModel noteDatabaseModel) async {
    Database _database = await _createDB();
    return await _database.rawInsert(
        'INSERT INTO tasks(content,status) values("${noteDatabaseModel.content}", "today")');
    // return await _database!.insert('tasks', noteDatabaseModel.toJason());
  }
}

执行sql:

class TodayPage extends StatelessWidget {
  const TodayPage();
  @override
  Widget build(BuildContext context) {
      DBHelper dbHelper=DBHelper();

     return  MaterialButton(
                      onPressed: () async{
                        // DBHelper _db = DBHelper.instance;
                      
                        NoteDatabaseModel note=NoteDatabaseModel(content: "hey",status: "today");
                         int x= await dbHelper.
                         insertNodeToDB(note);
                       print("\n print all Data ${  await dbHelper.
                         insertNodeToDB(note)}\n");
                      },
                      child: const Text("Click Me"),
                      ),
                    ),
          ],
    ); 
  }
}

Answer 1

你认为这行是做什么的：

if (_db == null) return _db!;

如果非要我猜的话，我会说应该是：

if (_db != null) return _db;

未处理的异常：空值检查运算符用于空值 E/flutter (28057)：#0 DBHelper._createDB

Unhandled Exception: Null check operator used on a null value E/flutter (28057): #0 DBHelper._createDB

dart

flutter

sqflite

dart-null-safety