This happened when I tried to close a separate image that I created as a CLONE OF A PREVIEW on a main image. I use this technique to speed up previewing my MMT settings because using a preview that is still attached to the main image to preview MMT settings seems to want to process the entire main image (in cache) and seems slower than creating a separate preview image. The problem with using this workaround now seems to be that Mask preview enabling/disabling (on the separate-image preview) is not working consistently when previewing MMT changes, and when I go to close the clone of the preview, I get this error message.
Restarting the application does not solve the problem.
