After creating a new namespace on theresnothingbetterwiki, recent changes broke T10093, pages were not retrievable T10071, and we got fun errors like Fatal exception of type "Wikimedia\Assert\ParameterAssertionException". User facing settings shouldn't be able to create fatal exceptions on the PHP level.

Before creating the namespace, ManageWiki needs to check that there don't exist names already that clash with the new namespace. You can choose to either reject the change, or fix the pages into the new namespace. Either is better than the current behavior.