Setting a root TreeItem with a null value stops the TreeTableView from showing the root's children.

This can be reproduced with the Monkey Tester
(TreeTableView page -> change root)
In the Monkey Tester, the issue can be "fixed" by re-loading the Skin (Skin -> Set New Skin) from the main menu.

Using the attached code example:
- run example
- expand root
- press "Change Root" button
- observe the child items not appearing when the root is expanded

See comments in lines 64-68.

NOTES:
1. this problem does not exist in the TreeView.
2. the bug also affects selection: set single selection mode, select row, collapse and expand root, select another row