Tolerate GetRoot() returning null.

content/browser/accessibility/browser_accessibility_manager.cc

 // 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.h"
 
 #include "base/logging.h"
 #include "content/browser/accessibility/browser_accessibility.h"
 #include "content/common/accessibility_messages.h"
 #include "ui/accessibility/ax_tree_serializer.h"
@@ skipping 111 matching lines @@
 ui::AXTreeUpdate BrowserAccessibilityManager::GetEmptyDocument() {
   ui::AXNodeData empty_document;
   empty_document.id = 0;
   empty_document.role = ui::AX_ROLE_ROOT_WEB_AREA;
   ui::AXTreeUpdate update;
   update.nodes.push_back(empty_document);
   return update;
 }
 
 BrowserAccessibility* BrowserAccessibilityManager::GetRoot() {
-  return GetFromAXNode(tree_->root());
+  // tree_->root() can be null during AXTreeDelegate callbacks.
+  ui::AXNode* root = tree_->root();
+  return root ? GetFromAXNode(root) : nullptr;
 }
 
 BrowserAccessibility* BrowserAccessibilityManager::GetFromAXNode(
     ui::AXNode* node) {
   return GetFromID(node->id());
 }
 
 BrowserAccessibility* BrowserAccessibilityManager::GetFromID(int32 id) {
   base::hash_map<int32, BrowserAccessibility*>::iterator iter =
       id_wrapper_map_.find(id);
@@ skipping 336 matching lines @@
 ui::AXTreeUpdate BrowserAccessibilityManager::SnapshotAXTreeForTesting() {
   scoped_ptr<ui::AXTreeSource<const ui::AXNode*> > tree_source(
       tree_->CreateTreeSource());
   ui::AXTreeSerializer<const ui::AXNode*> serializer(tree_source.get());
   ui::AXTreeUpdate update;
   serializer.SerializeChanges(tree_->root(), &update);
   return update;
 }
 
 }  // namespace content