Fix an old crash [1] when attempting to create an annotation if groups have not
yet loaded. In this case there is no focused group for the new annotation to be
associated with.

The handling for now is to just ignore the request and delete the highlight in
the guest, similar to how we handle the case where the user is not logged in,
except we don't show the login panel.

[1] https://hypothesis.sentry.io/issues/4412536210/