Studio UI lets you create a Content item as a child of another Content item that is not a container.

Steps to reproduce:
1. In Platform UI go to a Content item of a Type that is not a container (e.g. a Place in Studio Demo).
2. Check that you cannot create another content under it in Platform UI (the button is inactive)
3. Go to Studio UI Edit mode for this content.
4. Click +Create and choose either Landing Page or Content. You create any Content item normally.
(Additionally) 5. Go back to Platform UI and show the content tree. You will see a caret next to the 'illegal' parent, but you cannot expand it.

Tested on Studio Demo.