PopupMenuItem tap上未显示Flutter showDialog

PopupMenuButton(
  itemBuilder: (BuildContext context) {
    return <PopupMenuEntry>[
      PopupMenuItem(
        value: 1,
        child: Text('选项1'),
      ),
      PopupMenuItem(
        value: 2,
        child: Text('选项2'),
      ),
    ];
  },
  onSelected: (value) {
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: Text('对话框标题'),
          content: Text('对话框内容'),
          actions: <Widget>[
            FlatButton(
              child: Text('确定'),
              onPressed: () {
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
  },
)