Prerequisites:

1. Two non-container content items

Steps:

1. Go to Content > Content Structure > content_item_name
2. Click Move on the right action bar
3. Select second content item
4. Click Confirm

Actual result:

Error bar appears on the bottom:

Argument '$newParentLocation' is invalid: Cannot move to a parent that is not a container

Expected result:

You cannot select non-container content in UDW.