[TableTreeView] TreeTableView's selectedItems reports the addition of null items.

There are some cases where a TreeTableView's selectedItems will report the addition of null items. Here is a sample case:

https://gist.github.com/ryanjaeb/dc396c57614fa8c3e1a7

Steps to reproduce:

1. Run the example using 8u20-b17.
2. Sort the "name" column twice so the order is "two, three, one".
3. Expand item "two".

You'll get the following output.

// step 1
Change event...
 Change:
  Added: one

// step 2
Change event...
 Change:
Change event...
 Change:
  Added: one
Change event...
 Change:
Change event...
 Change:
  Added: one
Change event...
 Change:
Change event...
 Change:
  Added: one
Change event...
 Change:
Change event...
 Change:
  Added: one

// step 3
Change event...
 Change:
Change event...
 Change:
  Added: one
Change event...
 Change:
Change event...
 Change:
  WARNING: Added item was null.

Note the reported addition of a null item. This does not happen with 8u5:

1. Run the same example with 8u5.
2. Ensure "one" is selected.
3. Sort the "name" column twice so the order is "two, three, one".
4. Expand the "two" item.

You'll get the following output:

// step 1
// step 2
Change event...
 Change:
  Added: one

// step 3
Change event...
 Change:
Change event...
 Change:
  Added: one
Change event...
 Change:
Change event...
 Change:
  Added: one

// step 4
Change event...
 Change:
Change event...
 Change:
  Added: one

Note the difference. Also note there are fewer events using 8u5.