I clicked submit too soon. The attached patch inhibits closing of the Editor when the dialog box appears. It also sets the parent argument so that Tkinter on Windows actually presents a modal dialog box.

What's happening is the the dialog box enters a nested Tk eventloop which allows the editor window to be closed by the window manager via clicking "X". That re-enters the maybesave dialog. The first one unwinds and closes the editor window, setting its internal references to None. The initial maybesave resumes execution with the rug pulled out from beneath its feet and raises errors.