Quitting a JavaFX app that uses a child stage (two windows) results in the following exception:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at com.sun.javafx.tk.quantum.WindowStage.isVisible(WindowStage.java:422)
at com.sun.javafx.tk.quantum.GlassStage.requestClosingAllWindows(GlassStage.java:169)
at com.sun.javafx.tk.quantum.QuantumToolkit$18.handleQuitAction(QuantumToolkit.java:454)
at com.sun.glass.ui.mac.MacApplication$6.action(MacApplication.java:152)