let "getImageAttribute" deal with null costumes

HISTORY.md

@@ -64,6 +64,7 @@
 
 ### 2019-04-09
 * Blocks, Objects, Threads: new "getImageAttribute" reporter primitive
+* Objects, Threads: let "getImageAttribute" deal with null costumes 
 
 ### 2019-04-08
 * Blocks, Objects, Threads: new "getSoundAttribute" reporter primitive

src/objects.js

@@ -8910,14 +8910,19 @@ Costume.prototype.rasterized = function () {
 };
 
 Costume.prototype.pixels = function () {
-    var i,
-        pixels = [],
-        src = this.contents.getContext('2d').getImageData(
-            0,
-            0,
-            this.contents.width,
-            this.contents.height
-        );
+    var pixels = [],
+        src,
+        i;
+
+    if (!this.contents.width || !this.contents.height) {
+        return pixels;
+    }
+    src = this.contents.getContext('2d').getImageData(
+        0,
+        0,
+        this.contents.width,
+        this.contents.height
+    );
     for (i = 0; i < src.data.length; i += 4) {
         pixels.push(new List([
             src.data[i],

src/threads.js

@@ -4345,7 +4345,7 @@ Process.prototype.reportGetImageAttribute = function (choice, name) {
                     this.blockReceiver().costumes.asArray(),
                     function (c) {return c.name === name.toString(); }
                 )
-            ),
+            ) || new Costume(),
         option = this.inputOption(choice);
 
     switch (option) {