[TreeTable] TreeTable item selection causes a NullPointerException (regression).

Here is an example of the issue:

https://gist.github.com/ryanjaeb/baa6d0d20df29fce56d6

Steps to reproduce:

1. Run the example.
2. Click on the "one" item to select it.

A NPE will be thrown. This does not happen in 8u5 or 8u17. It's worth noting that selecting the item via code does not cause a NPE. I have to select the item via mouse click to see the issue.

This is my first try at testing something with a local OpenJFX build. My testing should be against revision 498c76f90ebf, but I'm not 100% positive I got everything right on the first try.