I was seeing this too, after an upgrade from 2.2.1 to 2.3.1 - and I can report that I was only seeing the error when I specified “show existing files”.
So, I decided to test if this only occurred if there were no associated files, I discovered when trying to upload images (which were the files I was working with) through the normal Publish/Edit interface, I was getting an error that the thumbnail directories were not writable.
When I changed the permission of the thumbnail directories, the error went away.
So this is likely a thumbnail directory permissions problem.