Crash when user do not select any action in the action list

[andybonnetto]

This situation is not taken into account in the code

return callback(*args, **kwargs)
  File "annotator.py", line 591, in main
    window = MainWindow(
  File "annotator.py", line 99, in __init__
    self.run_video(self.multiview)
  File "annotator.py", line 172, in run_video
    self.run_viewer(
  File "annotator.py", line 238, in run_viewer
    self.viewer = Viewer(
  File "...\dlc2action_annotation\widgets\viewer.py", line 244, in __init__
    self.initialize_cats()
  File "...\dlc2action_annotation\widgets\viewer.py", line 589, in initialize_cats
    for i, a in v.items():
AttributeError: 'list' object has no attribute 'items'