fix: formations list dropdown

[thomas-advantitge]

The formations dropdown is not displaying any formations anymore due to a NameError:

Traceback (most recent call last):
  File "~/Library/Application Support/Blender/4.0/scripts/addons/vendor/skybrush/sbstudio/plugin/props/formation.py", line 10, in _is_formation
    return is_formation(object)
  File "~/Library/Application Support/Blender/4.0/scripts/addons/vendor/skybrush/sbstudio/plugin/model/formation.py", line 249, in is_formation
    if not isinstance(object, Collection):
NameError: name 'Collection' is not defined. Did you mean: 'Collections'?

caused by 57a0c50#diff-5d9f0714c7b9ba49b973103efbe8b635c8527ea849bb112944d2a6e8011243b0R20

[ntamas]

Thanks a lot!