In a SB context, styling a MenuButton as follows, I get the NPE below.
I tried to reproduce in a test case, with thte same styling, without success.

/* Style class applied to MenuButton */
.cog-button {
     /*we make the padding the same as button for correct vertical alignment*/
    -fx-padding: 0.333333em 0.583em 0.333333em 0.583em; /* 4 7 4 7 */
    -fx-background-color: null;
    -fx-background-image: url("../images/cog.png");
    -fx-background-position: center;
    -fx-background-repeat: no-repeat;
    -fx-background-insets: 0;
}
.cog-button > .arrow-button {
visibility: false;
}
.cog-button:hover {
    -fx-background-image: url("../images/cog-hover.png");
}
.cog-button:focused {
    -fx-background-image: url("../images/cog-focused.png");
}





java.lang.NullPointerException
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:827)
at com.sun.javafx.sg.prism.NGNode.renderOpacity(NGNode.java:677)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:404)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:412)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:412)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.renderForClip(NGNode.java:633)
at com.sun.javafx.sg.prism.NGNode.renderRectClip(NGNode.java:527)
at com.sun.javafx.sg.prism.NGNode.renderClip(NGNode.java:553)
at com.sun.javafx.sg.prism.NGNode.access$000(NGNode.java:69)
at com.sun.javafx.sg.prism.NGNode$CacheFilter.impl_renderNodeToCache(NGNode.java:806)
at com.sun.javafx.sg.BaseCacheFilter.render(BaseCacheFilter.java:238)
at com.sun.javafx.sg.prism.NGNode$CacheFilter.render(NGNode.java:753)
at com.sun.javafx.sg.prism.NGNode.renderCached(NGNode.java:694)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:406)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:412)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:412)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.renderForClip(NGNode.java:633)
at com.sun.javafx.sg.prism.NGNode.renderRectClip(NGNode.java:527)
at com.sun.javafx.sg.prism.NGNode.renderClip(NGNode.java:553)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:408)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.renderForClip(NGNode.java:633)
at com.sun.javafx.sg.prism.NGNode.renderRectClip(NGNode.java:527)
at com.sun.javafx.sg.prism.NGNode.renderClip(NGNode.java:553)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:408)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.renderForClip(NGNode.java:633)
at com.sun.javafx.sg.prism.NGNode.renderRectClip(NGNode.java:527)
at com.sun.javafx.sg.prism.NGNode.renderClip(NGNode.java:553)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:408)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.renderForClip(NGNode.java:633)
at com.sun.javafx.sg.prism.NGNode.renderRectClip(NGNode.java:527)
at com.sun.javafx.sg.prism.NGNode.renderClip(NGNode.java:553)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:408)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.sg.prism.NGGroup.renderChildren(NGGroup.java:237)
at com.sun.javafx.sg.prism.NGGroup.renderContent(NGGroup.java:203)
at com.sun.javafx.sg.prism.NGRegion.renderContent(NGRegion.java:1254)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:412)
at com.sun.javafx.sg.prism.NGNode.doRender(NGNode.java:69)
at com.sun.javafx.sg.BaseNode.render(BaseNode.java:1283)
at com.sun.javafx.tk.quantum.ViewPainter.doPaint(ViewPainter.java:99)
at com.sun.javafx.tk.quantum.AbstractPainter.paintImpl(AbstractPainter.java:202)
at com.sun.javafx.tk.quantum.PresentingPainter.run(PresentingPainter.java:95)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:304)
at com.sun.prism.render.RenderJob.run(RenderJob.java:60)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at com.sun.javafx.tk.quantum.QuantumRenderer$PipelineRunnable.run(QuantumRenderer.java:129)
at java.lang.Thread.run(Thread.java:724)
D3D Vram Pool: 6,464,436 used (2.4%), 6,464,436 managed (2.4%), 268,435,456 total
41 total resources being managed
4 permanent resources (9.8%)
3 resources locked (7.3%)
31 resources contain interesting data (75.6%)
0 resources disappeared (0.0%)