small NPE fix

src/main/java/plugins/nherve/colorpickerthreshold/ColorPickerThreshold.java

@@ -499,7 +499,7 @@ public class ColorPickerThreshold extends PainterManagerSingletonPlugin<ColorPic
 		private int[] getClickedColor(Point pt) throws SignatureException {
 			IcyBufferedImage image = getCurrentSequence().getFirstImage();
 			int[] c = null;
-			if (image.isInside(pt)) {
+			if ((image != null) && image.isInside(pt)) {
 				int x = (int) pt.getX();
 				int y = (int) pt.getY();
 				c = ColorSpaceTools.getColorComponentsI_0_255(image, ColorSpaceTools.RGB, x, y);