When deselecting a TreeItem, the Change.getRemoved() contains a null entry. Instead it should contain only the item that was deselected.

Steps:
1. Run the attached test program
2. Select child item of root
3.Ctrl/Command click the same item to deselect it
-> Observe the NPE that occurs because Change.getRemoved() contains a null entry

The issue was also discussed in the review of JDK-8189354 at PR#480.
The PR#480 had a change in MultipleModelSelectionModelBase.java that could fix this issue but it was reverted to limit the scope PR#480
The reverted commit: https://github.com/openjdk/jfx/pull/480/commits/923f8d74039780065b02ac23d605cc010fde43b3