Tolerate GetRoot() returning null.

content/browser/accessibility/browser_accessibility_manager_mac.mm

 // Copyright (c) 2012 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.
 
 #include "content/browser/accessibility/browser_accessibility_manager_mac.h"
 
 #import "base/logging.h"
 #import "content/browser/accessibility/browser_accessibility_cocoa.h"
 #import "content/browser/accessibility/browser_accessibility_mac.h"
 #include "content/common/accessibility_messages.h"
@@ skipping 170 matching lines @@
   if (!created_live_region)
     return;
 
   // This code is to work around a bug in VoiceOver, where a new live
   // region that gets added is ignored. VoiceOver seems to only scan the
   // page for live regions once. By recreating the NSAccessibility
   // object for the root of the tree, we force VoiceOver to clear out its
   // internal state and find newly-added live regions this time.
   BrowserAccessibilityMac* root =
       static_cast<BrowserAccessibilityMac*>(GetRoot());
-  root->RecreateNativeObject();
-  NotifyAccessibilityEvent(ui::AX_EVENT_CHILDREN_CHANGED, root);
+  if (root) {
+    root->RecreateNativeObject();
+    NotifyAccessibilityEvent(ui::AX_EVENT_CHILDREN_CHANGED, root);
+  }
 }
 
 }  // namespace content