Create a simple TreeView (I used FileView example from Jonathan).
Collapse the root node and double click in the free area under the root node

java.lang.NullPointerException
at com.sun.javafx.scene.control.behavior.TreeCellBehavior.simpleSelect(TreeCellBehavior.java:76)
at com.sun.javafx.scene.control.behavior.TreeCellBehavior.mouseReleased(TreeCellBehavior.java:36)
at com.sun.javafx.scene.control.skin.SkinBase$MouseHandler.handle(SkinBase.java:251)
at com.sun.javafx.scene.control.skin.SkinBase$MouseHandler.handle(SkinBase.java:244)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:59)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchBubblingEvent(DefaultEventDispatcher.java:173)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchBubblingEvent(DefaultEventDispatcher.java:157)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchEvent(DefaultEventDispatcher.java:120)
at javafx.scene.Node$38.dispatchEvent(Node.java:4904)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchEvent(DefaultEventDispatcher.java:118)
at javafx.scene.Node$38.dispatchEvent(Node.java:4904)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchEvent(DefaultEventDispatcher.java:118)
at javafx.scene.Node$38.dispatchEvent(Node.java:4904)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchEvent(DefaultEventDispatcher.java:118)
at javafx.scene.Node$38.dispatchEvent(Node.java:4904)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.DefaultEventDispatcher.dispatchEvent(DefaultEventDispatcher.java:118)
at javafx.scene.Node$38.dispatchEvent(Node.java:4904)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at com.sun.javafx.event.EventRedirector.dispatchEvent(EventRedirector.java:93)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:92)
at javafx.event.Event.fireEventImpl(Event.java:172)
at javafx.event.Event.fireEvent(Event.java:157)
at javafx.scene.Node.fireEvent(Node.java:5009)
at javafx.scene.Scene$MouseHandler.process(Scene.java:2095)
at javafx.scene.Scene$MouseHandler.process(Scene.java:1930)
at javafx.scene.Scene$MouseHandler.access$1000(Scene.java:1902)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1019)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:1547)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:122)
at com.sun.glass.ui.View.handleMouseEvent(View.java:254)
at com.sun.glass.ui.View.notifyMouse(View.java:511)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
at com.sun.glass.ui.win.WinApplication$2.run(WinApplication.java:51)
at java.lang.Thread.run(Thread.java:619)