how to create a hierarchical list of lists? (aka Tree)

Looking at the Notepadv3 tutorial, I am trying to create a list of
lists - e.g. a tree.
However, I get an error "List views cant have UNSPECIFIED size"

Here is my (modified) notes_list.xml:

<?xml version="1.0" encoding="utf-8"?>

Any help appreciated.
thanks,
Anil