update method of TransferViewSet produces an error

[helrond]

Describe the bug

POST requests to TransferViewSet produce an error:
join() argument must be str or bytes, not \'NoneType\'

Expected behavior

POST requests SHOULD NOT trigger an error message but SHOULD trigger a run of the Aurora CleanupRoutine.

Additional context

I am pretty sure this is happening because the view is expecting an "identifier" to be passed with the request data. HOWEVER, I'm not sure that it's necessary to pass this data, because it should be possible to just get the machine_file_identifier based on the model instance which is being targeted. IOW, I think lines 92 and 93 should look like:

transfer = get_object_or_404(Transfer, pk=pk)
CleanupRoutine().run(transfer.machine_file_identifier)

You might need to import get_object_or_404 from django.shortcuts.

Tests should also be looked at