Gtk: NPE on extra mouse buttons click

Steps to reproduce:

Just click with extra mouse button(such as mouse4, mouse5) on any javafx application.

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.scene.Scene$ClickCounter.access$7600(Scene.java:2955)
at javafx.scene.Scene$ClickGenerator.preProcess(Scene.java:3051)
at javafx.scene.Scene$ClickGenerator.access$8400(Scene.java:3021)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3275)
at javafx.scene.Scene$MouseHandler.process(Scene.java:3137)
at javafx.scene.Scene$MouseHandler.access$1900(Scene.java:3089)
at javafx.scene.Scene.impl_processMouseEvent(Scene.java:1529)
at javafx.scene.Scene$ScenePeerListener.mouseEvent(Scene.java:2231)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleMouseEvent(GlassViewEventHandler.java:228)
at com.sun.glass.ui.View.handleMouseEvent(View.java:528)
at com.sun.glass.ui.View.notifyMouse(View.java:925)
at com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at com.sun.glass.ui.gtk.GtkApplication$3$1.run(GtkApplication.java:82)
at java.lang.Thread.run(Thread.java:722)