There is a suspicious condition in the 'com.sun.java.swing.plaf.gtk.GTKPainter#paintPopupMenuBackground' method:

            BufferedImage img = ENGINE.finishPainting();
            if(!isHW) {
                int border = img.getRGB(0, h / 2);
                if (img != null && border == img.getRGB(w / 2, h / 2)) {

'img != null' condition is always true, because it's only checked after 'img.getRGB` method is called - it means img can't be 'null'.