NullPointerException from StyleManager

From running ChoiceBox sample in Ensemble:

     [java] Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
     [java] at com.sun.javafx.css.StyleManager.forget(StyleManager.java:388)
     [java] at javafx.stage.Window$9.invalidated(Window.java:779)
     [java] at javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:107)
     [java] at javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:141)
     [java] at javafx.stage.Window.setShowing(Window.java:809)
     [java] at javafx.stage.Window.hide(Window.java:834)
     [java] at javafx.stage.PopupWindow.hide(PopupWindow.java:416)
     [java] at javafx.scene.control.ContextMenu.hide(ContextMenu.java:294)