[DevTools] Cleanup DOMExtension.js.

third_party/WebKit/Source/devtools/front_end/ui/UIUtils.js

 /*
  * Copyright (C) 2011 Google Inc.  All rights reserved.
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2007 Matt Lilek (pewtermoose@gmail.com).
  * Copyright (C) 2009 Joseph Pecoraro
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
@@ skipping 780 matching lines @@
 {
     if (event.target.document.nodeType === Node.DOCUMENT_NODE)
         document.body.classList.add("inactive");
 }
 
 /**
  * @param {!Event} event
  */
 WebInspector._focusChanged = function(event)
 {
-    var node = event.deepActiveElement();
-    WebInspector.Widget.focusWidgetForNode(node);
+    var document = event.target && event.target.ownerDocument;
+    var element = document ? document.deepActiveElement() : null;
+    WebInspector.Widget.focusWidgetForNode(element);
 }
 
 /**
  * @param {!Element} element
  * @constructor
  */
 WebInspector.ElementFocusRestorer = function(element)
 {
     this._element = element;
     this._previous = element.ownerDocument.deepActiveElement();
@@ skipping 1143 matching lines @@
  * @param {string} title
  * @return {!Element}
  */
 WebInspector.linkifyDocumentationURLAsNode = function(article, title)
 {
     return WebInspector.linkifyURLAsNode("https://developers.google.com/web/tools/chrome-devtools/" + article, title, undefined, true);
 }
 
 /** @type {!WebInspector.ThemeSupport} */
 WebInspector.themeSupport;