Unexpected error occurs after submitting Settings form without selecting a redirect location when returning to edit mode after publishing.

Steps:
1. Add Form block.
2. Fill mandatory fields.
3. Publish.
4. Switch to edit mode.
5. Open settings.
6. In Confirmation section switch from Thank you message to Redirect to location.
7. Do not select a location.
8. Click Save form.
Actual: Unexpected error occurs.
Expected: Select button is set to mandatory.