treeview crashes

Hi there I am trying to understand how the treeview example works as posted by Derez - any attempts I make at debugging (using either log or toastmessageshow) result in crashing out.

Specifically I have not understood how how the btn_click event knows which node it is related to e.g if my root has ten items and the fourth item is expandable how is the data passed that says to the btn_click event that it is the fourth item?

Probably missing something very simple but any help appreciated

thanks