When destroying the command stack, the commands stored are deleted in the insertion order which is a real pain in the ass when some commands depends on previously stored other ones. Say I have a command which create a document then I have an other one which create a element and add it to the previously created document. If the undo stack delete each of its stored commands in the insertion order the document will be deleted before the element. This is not cool because the element can't notify its document about its own deletion without crashing the program.