xamarin使用Rg.Plugins.Popup.Pages报Specified cast is not valid

1、在多个弹窗中使用了如下代码：

PopupNavigation.Instance.Popped += (s, ags) =>
{
	var page = (FileManagerPage)ags.Page;
	//......
};

2、调整为如下：

private private EventHandler<PopupNavigationEventArgs> popped;
//...

popped += (s, ags) =>
{
	PopupNavigation.Instance.Popped -= popped;
	var page = (FileManagerPage)ags.Page;
	//......
};

PopupNavigation.Instance.Popped += popped;