Closed paragraph validation error: This value should not be null

Steps to reproduce:
1. Add a paragraph type with required text field
2. Add paragraph field to basic page and use the experimental widget
3. Create a page and add paragraph with title
4. Add another paragraph and leave title empty
5. Click collapse all or edit the first paragraph (to collapse the second paragraph without title)
6. Click save

Expected result: Field validation fails with a message "title field is required".

Actual result: Field validation fails with a message "This value should not be null".