fix unable to add new favorite because one listed no longer exists

[4www]

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[oskarrough]

hey, what do we do with this one?

[4www]

I think it works.
Would be so nice to have a way to write automated tests for rules, but would be a huge system.

I think this element completes the rule a little bit. But we might as well test the solution with the firebase function + cleanup script first.

When this works , we can close or merge this PR.