为什么PopupWindowAction类在Prism 6中的对话框中难以维持模态模式？

I use PopupWindowAction class for creation of modal dialog windows (customized and predefined) in my Prism 6 WPF modular application with Unity. (I have Windows 10 on my computer and use MS VS 2015 Professional as development environment.) This is for customized modal dialog:I use PopupWindowAction class for creation of m