FOR UPDATE cannot be applied to the nullable side of an outer join on Postgres Django app

I'm trying to create a Django app for a course to learn it and I use this repo to practice: https://github.com/hltcoe/turkle

However, I ran into an issue where I keep on getting this error. Note that the issue only exists when I use postgres.

NotSupportedError at /batch/1/accept_next_task/
FOR UPDATE cannot be applied to the nullable side of an outer join