Sometimes when I try to undo, it will say please close the image editor before undoing again. You first edit a frame then go to another, and go back to the previous frame you were editing. Once this is done, you try to undo any previous modifications you made, and it will prompt this message. You are forced to close out of the editor.
It would be nice to have a separate undo/redo for each individual frame of the animation instead of just one set.
I still believe there should be a prompt to save changes though. Undoing several times is a tedious task to do.