fix: Avoid exception when migrating read states

[angonz]

When a thread is deleted, a read state may be retained pointing to a non-existent thread. This may cause a DoesNotExist exception during migration of read
states.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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[Kelketek]

@angonz Could you remove the .idea directory from this PR? Feel free to add it to the .gitignore since it's a very common IDE scratch directory.

[Kelketek]

@angonz This isn't mentioned in the PR description, nor in #177 . Is it intended to be included, or is this something specific to your use case?

[Kelketek]

Oh! My apologies, I didn't realize this PR was closed, @angonz -- since the issue was still open and linked here. Was this resolved some other way? I'm still getting the errors on my end, so I went to look over your solution.