oppia: Server error: AttributeError: 'NoneType' object has no attribute 'num_starts_v2'

This error is being repeatedly logged on stackdriver. It has occurred over 3500 times in the span on 24 hours. This looks like a serious bug, and would need to be handled ASAP.

AttributeError: 'NoneType' object has no attribute 'num_starts_v2'
at update_stats (/core/domain/stats_services.py:127)

/cc @brianrodri could you take a look?

General instructions There are no specific repro steps available for this bug report. The general procedure to fix server errors should be the following:

• Analyze the code in the file where the error occurred and come up with a hypothesis for the reason.
• Get the logic of the proposed fix validated by an Oppia team member (have this discussion on the issue thread).
• Make a PR that fixes the issue, then close the issue on merging the PR. (If the error reoccurs in production, the issue will be reopened for further investigation.)