I can not reproduce this. What I did was: in a shell I ran
java -jar Ensemble.jar

and did not use it for a while. Then I clicked the icon in the taskbar. From normal
Windows programs I am used that clicking on a taskbar icon will bring the
corresponding app to the front. This didn't happen for the Ensemble demo.
When I moved my mouse and looked into the shell output I saw multiple of those:

java.lang.NullPointerException
        at com.sun.prism.d3d.D3DSwapChain.present(D3DSwapChain.java:42)
        at com.sun.javafx.tk.quantum.PaintRunnable.run(PaintRunnable.java:241)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRunAndReset(Unknown Source)
        at java.util.concurrent.FutureTask.runAndReset(Unknown Source)
        at com.sun.prism.render.RenderJob.run(RenderJob.java:29)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at com.sun.javafx.tk.quantum.QuantumRenderer$ObservedRunnable.run(QuantumRenderer.java:70)
        at java.lang.Thread.run(Unknown Source)


Perhaps that stack trace is enough to identify where the NPE came from.