This makes sure the dialog will return None on Cancel.
git-svn-id: http://svn.robofab.com/branches/ufo3k@525 b5fa9d6c-a76f-4ffd-b3cb-f825fc41095c
git-svn-id: http://svn.robofab.com/branches/ufo3k@416 b5fa9d6c-a76f-4ffd-b3cb-f825fc41095c