Video-related error message in lessons with no video

In the last few days I have occasionally experienced an error when trying to save lessons.
The message I get is:

Video form errors:
Field “youtube_url”: Inserisci un indirizzo web valido (insert a valid url)
Field “language”: Questo campo è obbligatorio. (this field is required)

But there aren’t any videos in those lessons…

This happens when you click on a different page then use the “Back” button in your browser. It causes the grayed-out text to become solid text. Either refresh the page (before clicking Save) or just open up the Video section and delete the text that is there.