AttributeError: 'NoneType' object has no attribute 'tree_id'

[sentry]

For reference, this publishing issue could be related

Sentry Issue: STUDIO-FJ2

AttributeError: 'NoneType' object has no attribute 'tree_id'
(10 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "rest_framework/decorators.py", line 50, in handler
    return func(*args, **kwargs)
  File "contentcuration/views/base.py", line 338, in activate_channel_endpoint
    activate_channel(channel, request.user)
  File "contentcuration/api.py", line 74, in activate_channel
    user.check_channel_space(channel)
  File "contentcuration/models.py", line 255, in check_channel_space
    staging_tree_id = channel.staging_tree.tree_id

[bjester]

Addressed in #3855