The update center plugin site url can be saved empty, which leads to updating plugins no longer working.
Based from a ux aspect, that's an odd behavior, because you are able to change something in a way a core functionality becomes unavailable and you are unable to restore the original behavior, unless you dig deep enough or remember the original url.

The input should have some form of validation to forbid saving an empty update site url and an easy way to restore the plugin site url.