如何在TextFormField的onTap上添加弹出窗口

class PopupWindow extends StatefulWidget {
  @override
  _PopupWindowState createState() => _PopupWindowState();
}

class _PopupWindowState extends State<PopupWindow> {
  @override
  Widget build(BuildContext context) {
    return Container(
      // 弹出窗口的内容
    );
  }
}

TextFormField(
  onTap: () {
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return PopupWindow();
      },
    );
  },
  // 其他属性
)

class _PopupWindowState extends State<PopupWindow> {
  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: Text('弹出窗口'),
      content: Text('这是一个弹出窗口'),
      actions: [
        FlatButton(
          child: Text('关闭'),
          onPressed: () {
            Navigator.of(context).pop();
          },
        ),
      ],
    );
  }
}