I'll look into why. I fixed it so that it actually says the error (before it didn't), but there must be a problem with showing the error.

Edit: I see the problem, it had nothing to do with that. It's closing the save dialog window before it shows the error message window. When you close the error message, it then tries to go back to the window before it, but that window doesn't exist anymore. tpt++ is complicated.

Edit2: Fixed this, and also the crash when deleting stamps, with this commit