Reproducible with WebLauncher demo, just go to any site featuring animated images. http://rbc.ru will do.

     [java] java.lang.NullPointerException
     [java] at com.sun.prism.es2.ES2RTTexture.create(ES2RTTexture.java:71)
     [java] at com.sun.prism.es2.ES2ResourceFactory.createRTTexture(ES2ResourceFactory.java:49)
     [java] at com.sun.webpane.sg.prism.WCImgDecoderImpl$2.run(WCImgDecoderImpl.java:307)
     [java] at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
     [java] at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317)
     [java] at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150)
     [java] at com.sun.prism.render.RenderJob.run(RenderJob.java:29)
     [java] at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
     [java] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
     [java] at com.sun.javafx.tk.quantum.QuantumRenderer$ObservedRunnable.run(QuantumRenderer.java:67)
     [java] at java.lang.Thread.run(Thread.java:680)

The immediate cause seems that ES2Context.getGL() returns null. The initial request is (at WCImgDecoderImpl.java:307):
                    RTTexture txt = GraphicsPipeline.getDefaultResourceFactory().createRTTexture(imageWidth, imageHeight);