Lessons: "Add items from another page" link no longer functions

In Sakai 11 and 12, instructors could copy content from one Lessons page to another using "Add items from another page" (accessed by clicking "Reorder" in Lessons).

In Sakai 19 and 20, when you click Reorder and then "Add items from another page" nothing happens.

Steps to replicate on 19 and 20:
1. As an instructor, add the Lessons tool to a site.
2. Create 2 subpages in Lessons: for example, Week 1 and Week 2. (Or two top-level Lessons pages will produce same results.)
3. Add some content to the Week 1 subpage.
4. Go to the blank Week 2 subpage. Click Reorder at the top, then click "Add items from another page". NOTHING HAPPENS. You are returned to your blank Week 2 subpage.

Replicated using Chrome 74 and Firefox 60ESR (Win10) on these servers:

• https://qa19-mysql.nightly.sakaiproject.org/ (Build ee2b3cd7)
• Trunk/master (Build ab9f3d00) 

Expected behavior (what happens on 11 and 12):

• After following steps 1-3 above on Sakai 11 or 12...
• Go to the blank Week 2 subpage. Click Reorder at the top, then choose "Add items from another page".
• A list of existing pages appears. Select the Week 1 subpage. Then click "Use selected item".
• A list of all content items from Week 1 page is displayed. Click SAVE. All content from the Week 1 subpage is now copied onto the Week 2 subpage.