Add App State Button disabled for Doc Reference Types

I'm trying to add a "Cart" and "Favorites" app state to my yard sales app. The problem I'm facing is that when I choose any other type for the state it works but only "Document Reference" doesn't seem to work. The "Create" button disables when I choose Document reference.

This is an example of a "String" Type:

And This is an example of a "Document Reference" Type:

The button is disabled. Why is that the case and what can I do about it?