To reproduce: collapse/expand root several times until you get

Exception in thread "Thread-4" java.lang.NullPointerException
at com.sun.javafx.scene.control.skin.ProgressBarSkin$10.run(ProgressBarSkin.java:377)
at java.lang.Thread.run(Thread.java:724)

As a result root will be collapsed and progress bar cells will be rendered where they prevously were.