Attempt to fix Sentry errors

Issues:

- No replication steps
- No consistent errors (associated Sentry link in issue is not found)
- MRs not even primary cause of this type of error (it's mostly the Issuable Popover in Issues reading `issue`, not `mergeRequest`)

Given the exact error that _had been_ reported ("Cannot read properties of null (reading 'mergeRequest')", these tweaks may help prevent a handful (literally: 5-10 over two weeks) of errors.