切换窗口可见性会实现新窗口

I have a modularized application with Prism. A simple window (shell) is displayed. The shell contains a taskbar icon that invokes a command to toggle windows visibility. Clicking the TaskbarIcon creates a new instance of my shell instead of toggling the visibility of the original one. Does someone know why my code does not invoke the method on first shell?I have a modularized application with Prism. A