Steps to reproduce:
- Load https://beta.musicbrainz.org/work/create
- Enter a title
- Paste http://www.wikidata.org/wiki/Q1 into the URL field
- Clear the URL field (using ctrl/cmd-z or backspace or whatever)
At this point, it will complain that the URL field is required.
- Delete the URL field using the red x
Since the error has been fixed, the form should submit, but instead nothing happens.