I tried to display a tree with each of the treenodes having a checkbox. I looked at the demo workplace, but it seems your code is the same for "simple tree" and "tree with checkbox". But I figured out that I have to add the checkbox inside the treenode.
Works fine when entering a screen with this tree. But when leaving this screen, an exception on client side occurrs. The same happens in your demo workspace when clicking on "tree with checkbox" and the then leaving the screen again (click on some other element).