fixed pixel color sensing for stage pen trails

HISTORY.md

@@ -5,6 +5,7 @@
 ### 2019-01-04
 * Objects, Blocks, Threads: new feature/block: sense colors and sprites anywhere
 * updated German translation 
+* Objects: fixed pixel color sensing for stage pen trails 
 
 ### 2019-01-02
 * Objects, Blocks, Threads, GUI, Store: added third color dimension and transparency to pen

src/objects.js

@@ -6566,7 +6566,7 @@ StageMorph.prototype.getPixelColor = function (aPoint) {
     var point, context, data;
 	if (this.trailsCanvas) {
         point = aPoint.subtract(this.bounds.origin);
-        context = this.trailsCanvas.getContext('2d');
+        context = this.penTrailsMorph().image.getContext('2d');
         data = context.getImageData(point.x, point.y, 1, 1);
         if (data.data[3] === 0) {
         	return StageMorph.uber.getPixelColor.call(this, aPoint);