500 Error on GET /api/job-file-stage-groups/ on dev server

```
ERROR:django.request:Internal Server Error: /api/v2/job-file-stage-groups/,
referer: https://bespin-dev.gcb.duke.edu/api/v2/
... DJANGO and DRF Files
File "/app/data/models.py", line 259, in __str__, referer: https://bespin-dev.gcb.duke.edu/api/v2/
return "Job - pk: {} user: '{}' state: '{}' workflow_version.pk: {} ".format(self.pk, self.user,
self.get_state_display(), self.workflow_version.pk, ), referer: https://bespin-dev.gcb.duke.edu/api/v2/
AttributeError: 'NoneType' object has no attribute 'pk', referer: https://bespin-dev.gcb.duke.edu/api/v2/
, referer: https://bespin-dev.gcb.duke.edu/api/v2/
"GET /api/v2/job-file-stage-groups/ HTTP/1.1" 500 27
```

The error is occuring the the `Job.__str__` method.