Fix TypeError: path should be string, cannot be None
Fix TypeError: path should be string, cannot be None

Fix error when hitting cancel from the file dialog
caused by the selection returning None.

Signed-off-by: Dan Yeaw <2591e5f46f28d303f9dc027d475a5c60d8dea17a@yeaw.me>

amolenaar/gaphor,amolenaar/gaphor