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)
• Submit

At this point, it will complain that the URL field is required.

• Delete the URL field using the red x
• Submit

Since the error has been fixed, the form should submit, but instead nothing happens.