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

Source/devtools/front_end/timeline/TracingModel.js

 /*
  * Copyright 2014 The Chromium Authors. All rights reserved.
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 /**
  * @constructor
  */
 WebInspector.TracingModel = function()
@@ skipping 303 matching lines @@
     {
         this._tracingModel.tracingComplete();
     }
 }
 
 
 /**
  * @constructor
  * @param {string} category
  * @param {string} name
- * @param {string} phase
+ * @param {!WebInspector.TracingModel.Phase} phase
  * @param {number} startTime
- * @param {?WebInspector.TracingModel.Thread} thread
+ * @param {!WebInspector.TracingModel.Thread} thread
  */
 WebInspector.TracingModel.Event = function(category, name, phase, startTime, thread)
 {
+    /** @type {string} */
     this.category = category;
+    /** @type {string} */
     this.name = name;
+    /** @type {!WebInspector.TracingModel.Phase} */
     this.phase = phase;
+    /** @type {number} */
     this.startTime = startTime;
+    /** @type {!WebInspector.TracingModel.Thread} */
     this.thread = thread;
     this.args = {};
 
     /** @type {?string} */
     this.warning = null;
     /** @type {?WebInspector.TracingModel.Event} */
     this.initiator = null;
     /** @type {?Array.<!ConsoleAgent.CallFrame>} */
     this.stackTrace = null;
     /** @type {?Element} */
     this.previewElement = null;
     /** @type {?string} */
     this.imageURL = null;
     /** @type {number} */
     this.backendNodeId = 0;
 
     /** @type {number} */
     this.selfTime = 0;
 }
 
 /**
  * @param {!WebInspector.TracingManager.EventPayload} payload
- * @param {?WebInspector.TracingModel.Thread} thread
+ * @param {!WebInspector.TracingModel.Thread} thread
  * @return {!WebInspector.TracingModel.Event}
  */
 WebInspector.TracingModel.Event.fromPayload = function(payload, thread)
 {
-    var event = new WebInspector.TracingModel.Event(payload.cat, payload.name, payload.ph, payload.ts / 1000, thread);
+    var event = new WebInspector.TracingModel.Event(payload.cat, payload.name, /** @type {!WebInspector.TracingModel.Phase} */ (payload.ph), payload.ts / 1000, thread);
     if (payload.args)
         event.addArgs(payload.args);
     else
         console.error("Missing mandatory event argument 'args' at " + payload.ts / 1000);
     if (typeof payload.dur === "number")
         event.setEndTime((payload.ts + payload.dur) / 1000);
     if (payload.id)
         event.id = payload.id;
     return event;
 }
@@ skipping 69 matching lines @@
     // startTime's are equal, so both events got placed into the result array.
     return a.startTime - b.startTime || -1;
 }
 
 /**
  * @constructor
  * @extends {WebInspector.TracingModel.Event}
  * @param {string} category
  * @param {string} name
  * @param {number} startTime
- * @param {?WebInspector.TracingModel.Thread} thread
+ * @param {!WebInspector.TracingModel.Thread} thread
  */
 WebInspector.TracingModel.ObjectSnapshot = function(category, name, startTime, thread)
 {
     WebInspector.TracingModel.Event.call(this, category, name, WebInspector.TracingModel.Phase.SnapshotObject, startTime, thread);
 }
 
 /**
  * @param {!WebInspector.TracingManager.EventPayload} payload
- * @param {?WebInspector.TracingModel.Thread} thread
+ * @param {!WebInspector.TracingModel.Thread} thread
  * @return {!WebInspector.TracingModel.ObjectSnapshot}
  */
 WebInspector.TracingModel.ObjectSnapshot.fromPayload = function(payload, thread)
 {
     var snapshot = new WebInspector.TracingModel.ObjectSnapshot(payload.cat, payload.name, payload.ts / 1000, thread);
     if (payload.id)
         snapshot.id = payload.id;
     if (!payload.args || !payload.args["snapshot"]) {
         console.error("Missing mandatory 'snapshot' argument at " + payload.ts / 1000);
         return snapshot;
@@ skipping 398 matching lines @@
     /**
      * @return {!Array.<!WebInspector.TracingModel.Event>}
      */
     asyncEvents: function()
     {
         return this._asyncEvents;
     },
 
     __proto__: WebInspector.TracingModel.NamedObject.prototype
 }