由于您没有将其直接绑定到onPressed事件，因此需要调用该函数.试试这个:

Since you are not binding it directly to the onPressed event, you need to call the function. Try this:

    new FlatButton(
      onPressed: () {
        btnSi == null ? Navigator.of(context).pop(true):btnSi();
      },
      child: new Text("Si"),
    )