无法以编程方式将焦点设置到ExpansionTile内部的TextField

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

class _MyWidgetState extends State<MyWidget> {
  FocusNode _focusNode = FocusNode();

  @override
  Widget build(BuildContext context) {
    return ExpansionTile(
      title: Text('Expansion Tile'),
      onExpansionChanged: (bool expanded) {
        if (expanded) {
          FocusScope.of(context).requestFocus(_focusNode);
        }
      },
      children: [
        TextField(
          focusNode: _focusNode,
          decoration: InputDecoration(
            labelText: 'Enter text',
          ),
        ),
      ],
    );
  }
}