About unknown error and 502 bad gateway

[flyingtango]

After I submit the csv file to the backend, the page would jump to 502 Bad Gateway nginx/1.15.8. But I can find the upload file in /submition/instance/upload. Meanwhile, when I click View Score and the page shows Unknown error. The logs are below
File "./submission/views.py", line 178, in
dates = sorted(list({s.submitted_on.date() for s in submissions}))
AttributeError: 'NoneType' object has no attribute 'date'