When you send end() to an UndoManager, it starts to behave like a normal CompoundEdit, with Undo and Redo triggering every edit in the list, not just the last one. This makes it important that edits at and beyond the insertion index are cleared out and killed off. Unfortunately, right now this doesn't happen until the next call to addEdit().

This causes exceptions if:
*nested undo session starts
*changes happen
*at least one undo happens
*the temporary undomanager is end()ed
*and it is told to undo, BEFORE any more changes are added