fix List::length to work with stored CS10 projects

history.txt

@@ -2196,3 +2196,8 @@ ______
 * Threads: fixed #506, thanks @haritop, for both the report and for providing the fix!!
 * GUI: fixed #412 (incomplete sprite-removal)
 * GUI: fixed #507 (limit persistent block zoom to 12x), thanks Michael!
+* Morphic, GUI, Objects: fixed #508 (don’t popup empty menus), thanks Michael!
+
+140714
+------
+* Lists: make internal list ops iterative (instead of recursive), thanks, Brian!

lists.js

@@ -195,11 +195,11 @@ List.prototype.length = function () {
     if (this.isLinked) {
         var pair = this,
             result = 0;
-        while (pair.isLinked) {
+        while (pair && pair.isLinked) {
             result += 1;
             pair = pair.rest;
         }
-        return result + pair.contents.length;
+        return result + (pair ? pair.contents.length : 0);
     }
     return this.contents.length;
 };