I will update this to say that the black template IS working on the WP popup login screen when your session times out, but not on the main login page.
I am having the exact same issue. I can deactivate, but cannot uninstall. If I uninstall I get the same error you are getting and am locked out of my site. I have tried a dozen fixes for that error, and none work. I have spent many hours trying to figure this out and hope the developers respond to this!
Did you find a solution?
Thanks for any help.
