To reproduce:
1) Start Scene Builder 2.0
2) Open attached FXML (SplitPaneAndToolbar.fxml)
3) Preview -> Show Preview Window (see PreviewBeforeResizing.png)
4) Drag splitter to the left
    => left pane of the SplitPane is shrinker
    => "Everything" combo box disappear
    => the exception below is dumped in Scene Builder log : BUG
    => the Preview window is broken (document must be closed and reopened to try again)

java.lang.ArrayIndexOutOfBoundsException: -1
at java.util.ArrayList.elementData(ArrayList.java:403)
at java.util.ArrayList.get(ArrayList.java:416)
at com.sun.javafx.scene.control.skin.ToolBarSkin.addNodesToToolBar(ToolBarSkin.java:459)
at com.sun.javafx.scene.control.skin.ToolBarSkin.layoutChildren(ToolBarSkin.java:319)
at javafx.scene.control.Control.layoutChildren(Control.java:574)
at javafx.scene.Parent.layout(Parent.java:1076)
at javafx.scene.Parent.layout(Parent.java:1082)
at javafx.scene.Parent.layout(Parent.java:1082)
at javafx.scene.Parent.layout(Parent.java:1082)
at javafx.scene.Parent.layout(Parent.java:1082)
at javafx.scene.Scene.doLayoutPass(Scene.java:576)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2386)
at com.sun.javafx.tk.Toolkit$3.run(Toolkit.java:322)
at com.sun.javafx.tk.Toolkit$3.run(Toolkit.java:320)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:320)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:349)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:479)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:460)
at com.sun.javafx.tk.quantum.QuantumToolkit$13.run(QuantumToolkit.java:327)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)