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

Sentry Issue: ARKINDEX-BACKEND-34

AttributeError: 'NoneType' object has no attribute 'slug'
(4 additional frame(s) were not displayed)
...
  File "rest_framework/views.py", line 505, in dispatch
    response = self.handle_exception(exc)
  File "rest_framework/views.py", line 465, in handle_exception
    self.raise_uncaught_exception(exc)
  File "rest_framework/views.py", line 476, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 502, in dispatch
    response = handler(request, *args, **kwargs)
  File "arkindex/documents/api/ml.py", line 560, in put

Internal Server Error: /api/v1/classifications/1b96545a-58ae-4f48-b79d-c93a63c935ce/reject/