I was able to recreate this in a fresh web site in VS2012 Pro, although another pre-existing project (different than the one I first discovered this error on) has multiple validators on one page and yet is fine. I also tried a few different versions ranging from the most recent, 2019.1.215.45, back to 2015.2.826.45. This leads me to believe there is something else that is a factor here, as I doubt I'm the only person to have tried to close a RadWindow under these circumstances since 2015.
Fortunately this error can be resolved for now by simply disabling animations on my RadWindows in that project, but I would like to be able to re-enable them at some point.
This is my code (no code-behind used):
And this is the error I got on opening the window and clicking cancel:
Following the above stack trace, I went in to the _hide function and found the line (line 11 below) where the error was occurring, which was how I figured the animation was part of the issue: