flutter error:此小部件已卸载，因此State不再有上下文(应视为已失效)

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  String _text = 'Hello';

  @override
  void dispose() {
    // 在小部件被销毁时调用
    super.dispose();
  }

  void _updateText() {
    if (mounted) {
      // 检查小部件是否仍然在Flutter树中
      setState(() {
        _text = 'World';
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text(_text),
        RaisedButton(
          onPressed: _updateText,
          child: Text('Update Text'),
        ),
      ],
    );
  }
}