How can I only allow setting a field in directus, if another field is empty?

Question

In my Directus collection called link, I have two fields: page and url.
Only on at a time should be set.

I added a condition to the url field and set it to "Page is null". I did the same for page, where I set the rule to "Url is null".

But it doesn't do anything, creating a link while setting both url and page still works.