Because of missing validations in the projects model, you could
create a new project with a nil or empty import_url with mirror=true.
This would cause exceptions when
RepositoryUpdateMirrorWorker::UpdateMirrorError ran and attempted to
update one of these projects. This was made easier by the fact that
you could show the "Mirror repository" checkbox in the UI, check it,
hide it, and still have its value considered.

The missing validations have been added, and minor adjustments have
been made to the UI to improve the previous behaviour. A migration
was also added to update mirror=false in all projects with a nil or
empty import url that exists because of the described bug.