call helper.update() only if the helper has an update method

Author: vincentfretin

src/index.js

@@ -126,7 +126,8 @@ Inspector.prototype = {
       helper.visible = false;
       this.sceneHelpers.add(helper);
       this.helpers[object.uuid] = helper;
-      helper.update();
+      // SkeletonHelper doesn't have an update method
+      if (helper.update) helper.update();
     };
   })(),