Comment 25 for bug 1871898

Here's a PR which is based on looking at the database dump and making an educated guess as to what needs fixing. The logic in the PR is designed to cope with a relation claiming to still have units in scope but there are none. That normally can't happen but it could be that using --force in various ways partially cleaned up and left inconsistent data.

https://github.com/juju/juju/pull/12470

The fix can't be guaranteed to address the original issue reported when the bug was opened but hopefully closes another corner case.