DevTools: Fix exception on timeline clear

Source/devtools/front_end/timeline/TimelineModel.js

 /*
  * Copyright (C) 2012 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 1535 matching lines @@
  */
 WebInspector.TimelineModel.buildBottomUpTree = function(topDownTree, groupingCallback)
 {
     var buRoot = new WebInspector.TimelineModel.ProfileTreeNode();
     buRoot.selfTime = 0;
     buRoot.totalTime = 0;
     buRoot.name = WebInspector.UIString("Bottom-Up Chart");
     /** @type {!Map<string,!WebInspector.TimelineModel.ProfileTreeNode>} */
     buRoot.children = new Map();
     var nodesOnStack = /** @type {!Set<string>} */ (new Set());
-    topDownTree.children.forEach(processNode);
+    if (topDownTree.children)
+        topDownTree.children.forEach(processNode);
 
     /**
      * @param {!WebInspector.TimelineModel.ProfileTreeNode} tdNode
      */
     function processNode(tdNode)
     {
         var buParent = groupingCallback && groupingCallback(tdNode) || buRoot;
         appendNode(tdNode, buParent);
         var hadNode = nodesOnStack.has(tdNode.id);
         if (!hadNode)
@@ skipping 795 matching lines @@
         /** @type {!Object.<string, !Array.<!WebInspector.InvalidationTrackingEvent>>} */
         this._invalidations = {};
         /** @type {!Object.<number, !Array.<!WebInspector.InvalidationTrackingEvent>>} */
         this._invalidationsByNodeId = {};
 
         this._lastRecalcStyle = undefined;
         this._lastPaintWithLayer = undefined;
         this._didPaint = false;
     }
 }