Cancelling the file selection dialog throws an error and blocks RoboFont

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When cancelling the designspace file selection dialog, I get the following traceback and the sheet blocks RoboFont:

PyObjC: Converting exception to Objective-C:Traceback (most recent call last):
  File "/Applications/RoboFont Beta.app/Contents/Resources/lib/python3.12/vanilla/vanillaBase.py", line 539, in action_
    self.callback(sender)
  File "/Applications/RoboFont Beta.app/Contents/Resources/lib/python3.12/ezui/items/addRemoveButton.py", line 137, in _internalCallback
    self._addCallback(self)
  File "main.py", line 129, in fileTableAddRemoveButtonAddCallback
TypeError: 'NoneType' object is not iterable
PyObjC: Converting exception to Objective-C:Traceback (most recent call last):
  File "lib/doodleDelegate.pyc", line 99, in sendEvent_
  File "/Applications/RoboFont Beta.app/Contents/Resources/lib/python3.12/vanilla/vanillaBase.py", line 539, in action_
    self.callback(sender)
  File "/Applications/RoboFont Beta.app/Contents/Resources/lib/python3.12/ezui/items/addRemoveButton.py", line 137, in _internalCallback
    self._addCallback(self)
  File "main.py", line 129, in fileTableAddRemoveButtonAddCallback
TypeError: 'NoneType' object is not iterable