This is pretty niche, but here goes.
I have a model that returns a single row with a rich text field. I put that field in a field editor in a popup for editing. I also have an action on the model so that when it’s saved, all popups are closed. The action seems to create a conflict with the CKEditor used for rich text fields, because saving the record (which fires the close popups action) gives a console error:
ckeditor uncaught typeerror: cannot read property 'unselectable' of null
Removing the action eliminates the problem, which I’ve done for now.
This is in 5.21.8.