I found the hard way that I had to use FX.deferAction()
when calling back to JavaFX from JavaScript.

// The applet uses this to set the current Image shown
public function setCurrentPic(ndx:Integer) : Void {
  FX.deferAction(function(): Void {
      try {
          imageList.setIndex(ndx);
      } catch(ex: java.lang.Exception) {
          applet.eval("alert('{ex}')");
      }
  });
}

Without the deferAction() it sometimes would work, other times I would
get a SecurityException when trying to fetch the associated image off the net.
With deferAction it all seems to work fine.

I would have thought that this would be automagically done
by the JavaScript to JavaFX framework?