Key can't be accepted because of key with null fingerprint

[daniele-athome]

I bumped into a case where the was a regular record in the keys table with all correct data, and a more recent record with "null" fingerprint (as in "null" as a string) which makes accepting the key impossible (the red snackbar is stuck in a loop). Verify what triggered the "null" value in the first place because it couldn't be there.
I'm afraid this blocks the next release because it's a serious bug.

Note: happening when accepting a new user invitation (that is, unknown number) after running a sync.