[Forms] Forms created with an old version throw 500s

Forms created with v1.3.14 of WF that is now at the latest version cause the page they're attached to to throw a 500 server error.

The issue seems to appear from the fact that the slug of the form is `null.`
A workaround exists for the issue. If a user changes something in the form (and saves it), the slug is generated, and the form will work.

## Steps to reproduce
1. Start a WF instance with v1.3.14
2. Create a form in the admin
3. Upgrade the instance to the latest version (v1.6.7)
4. Create a new page
5. Attach the form to the new page
6. Publish & save the page
7. Open the page
8. The page opens with a 500 Server Error

## Expected
The page should open and show the form.
Otherwise, could you send the user a warning/error? When saving the page, the user should see an error banner. The banner should mention that a problem occurred because the form is incompatible.