I have deleted a project but GitLab gives me this message afterwards:
This project was scheduled for deletion, but failed with the following message: undefined method `merge_requests’ for nil:NilClass
The project is still listed in my dashboard.
Workaround
In settings, I renamed the recalcitrant project.
This allowed me to fork the original project again successfully and continue contributing as before.
My renamed project still exists and cannot be removed, but at least it’s out of the way.