Steps to reproduce

1. Go to Setting -> Theming
2. Change internal Link to föck.it (not my url)

Expected behaviour

Tell us what should happen:
It should let me save this link, since it is a valid link.

Actual behaviour

Tell us what happens instead:
It tells me, that it is a invalid Link

Operating system:
Any

Web server:
Apache2

Database:
MySQL

PHP version:
7.4

Nextcloud version: (see Nextcloud admin page)
20.0.4

Updated from an older Nextcloud/ownCloud or fresh install:
Fresh

Where did you install Nextcloud from:
nextcloud.com