Creating new WPF window within a MAF Add-In

Hello, 

I have a WPF application, which hosts a series of isolated plugins as documentedhere.  Everything seems to be working very well, only recently I tried to get one of my Add-Ins to create a child window (nothing particularly notable about the code!):

            Window w = new Window();
            w.Width = 100;
            w.Height = 100;
            w.Show();

When I subsequently shut this window down, the entire UI for the plug-in disappears!  I don't seem to get any exceptions that I can hook in to (I'm hooking in to the unhandled exception events on the AppDomain and Dispatcher).

Any suggestions or advice gratefully received!