I stumbled upon this page after having the same problem. Although I'm not sure this solution may help you it helped me and maybe can be useful to someone else.
My WPF application needed to run with elevated privileges and if I tried to run VS without elevated privileges my tests broke with the aforementioned error. What I did was simply restar VS as Administrator and the tests started working.
One thing I noticed was that when I was running without elevated privileges the Window.Framework was 'Win32' and the Window.Items was empty. After running VS as Administrator The Framework propery was 'Wpf' and the Items list was filled correctly. I don't know what is the real cause of the problem here. But it worked for me and maybe it'll work for someone else.