Server Error when clicking 'Manager' on a proofreading task

[mauromsl]

Describe the bug
In typesetting, when I click 'Manage' next to the proofreading task this goes to an error page

The corrected proofs for this article have just been returned from the typesetter, so I was clicking on this to try to access the request that was originally sent to the typesetter containing the proofreader's comments, in order to check that these had all been actioned. I figured out how to do this by clicking 'Admin' to access the comments, but just flagging the error message here (possibly occurring because the proofreading task is complete?) Thanks again

StackTrace:

ValueError: list.remove(x): x not in list
  File "django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 249, in _legacy_get_response
    response = self._get_response(request)
  File "django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "security/decorators.py", line 698, in wrapper
    return func(request, *args, **kwargs)
  File "security/decorators.py", line 63, in wrapper
    return func(request, *args, **kwargs)
  File "security/decorators.py", line 350, in wrapper
    return func(request, *args, **kwargs)
  File "/var/www/olh/src/plugins/typesetting/views.py", line 1025, in typesetting_manage_proofing_assignment
    assignment=assignment,
  File "/var/www/olh/src/plugins/typesetting/logic.py", line 76, in get_proofreaders
    current_proofer_pks.remove(assignment.proofreader.pk)

[ajrbyers]

This was fixed by fd7983d