NPE when rendering NGRegion when returning from screen lock

Note that you need the fix for RT-32636, which is under review, in order to see this bug, since that bug causes all rendering to cease.

To reproduce:

* Run any program with controls, e.g., "HelloButton"
* Go into screen lock using CTRL-ALT-DEL (or Windows-L)
* Return from screen lock by pressing ESC (or logging back in)

You will get the following NPE in the renderer which kills rendering:

java.lang.NullPointerException
        at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:627)
        at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:2028)
        at com.sun.javafx.sg.prism.NGNode.render(NGNode.java:1937)
        at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:225)
        at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:2028)
        at com.sun.javafx.sg.prism.NGNode.render(NGNode.java:1937)
        at com.sun.javafx.tk.quantum.ViewPainter.doPaint(ViewPainter.java:472)
        at com.sun.javafx.tk.quantum.ViewPainter.paintImpl(ViewPainter.java:324)
        at com.sun.javafx.tk.quantum.PresentingPainter.run(PresentingPainter.java:88)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:308)
        at com.sun.javafx.tk.RenderJob.run(RenderJob.java:58)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at com.sun.javafx.tk.quantum.QuantumRenderer$PipelineRunnable.run(QuantumRenderer.java:129)
        at java.lang.Thread.run(Thread.java:724)