Fix null pointer exception when attempting to load an image without the target

Source/devtools/front_end/timeline/TimelineFrameModel.js

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 641 matching lines @@
      */
     loadPicture: function(callback)
     {
         var target = this._event.thread.target();
         this._event.picture.requestObject(onGotObject);
         /**
          * @param {?Object} result
          */
         function onGotObject(result)
         {
-            if (!result || !result["skp64"]) {
+            if (!result || !result["skp64"] || !target) {
                 callback(null, null);
                 return;
             }
             var rect = result["params"] && result["params"]["layer_rect"];
             WebInspector.PaintProfilerSnapshot.load(target, result["skp64"], callback.bind(null, rect));
         }
     }
 };
 
 /**
  * @constructor
  */
 WebInspector.PendingFrame = function()
 {
     /** @type {!Object.<string, number>} */
     this.timeByCategory = {};
     /** @type {!Array.<!WebInspector.LayerPaintEvent>} */
     this.paints = [];
 }