In News content elements: "Add new record after this record" + button not working

Summary

When editing a news article, clicking the "Add new record after this record" plus button in the Content elements tab results in an "undefined error" message.

Steps to reproduce

1. SU to my user

2. Create a new news article

3. Add a content element using the "Create new" button

4. Save

5. Click the "Add new record after this record" plus button on the content element you created