Steps to Reproduce

1. Build a stateless tree with TreeModel using ITreeController.
2. set checkmark="true" and multiple="true" to enable multi-select mode.
3. open browser check any  checkbox.
4. un-check the checkbox you just checked.
5. NullPointerException should occur.

Current Result

NPE occur.

Note: NPE only occur when you `un-check a checkbox`, IOW occur when `the tree's selection count changes from 1 to 0`.

Expected Result

No exceptions.