I haven't played with the symbols in the tree control, but it sounds like from the error message that the item currently selected in the control can't have a symbol associated with it--like perhaps if nothing is selected? Can you post your code? That would help in troubleshooting.
In terms of your other questions, there isn't a tutorial on the thing, or a video. The interface is not particularly intuitive either. I have been playing with the thing fairly extensively and might be able to provide some pointers. What are you trying to do?
Mike...