Fix crash in widget destruction

ui/views/accessibility/ax_widget_obj_wrapper.cc

 // 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.
 
 #include "ui/views/accessibility/ax_widget_obj_wrapper.h"
 
 #include "base/strings/utf_string_conversions.h"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/views/accessibility/ax_aura_obj_cache.h"
 #include "ui/views/accessibility/ax_aura_obj_wrapper.h"
@@ skipping 14 matching lines @@
   }
   widget_ = NULL;
 }
 
 AXAuraObjWrapper* AXWidgetObjWrapper::GetParent() {
   return AXAuraObjCache::GetInstance()->GetOrCreate(widget_->GetNativeView());
 }
 
 void AXWidgetObjWrapper::GetChildren(
     std::vector<AXAuraObjWrapper*>* out_children) {
-  if (!widget_->IsVisible() || !widget_->GetRootView()->visible())
+  if (!widget_->IsVisible() || !widget_->GetRootView() ||
+      !widget_->GetRootView()->visible()) {
     return;
+  }
 
   out_children->push_back(
       AXAuraObjCache::GetInstance()->GetOrCreate(widget_->GetRootView()));
 }
 
 void AXWidgetObjWrapper::Serialize(ui::AXNodeData* out_node_data) {
   out_node_data->id = GetID();
   out_node_data->role = widget_->widget_delegate()->GetAccessibleWindowRole();
   out_node_data->AddStringAttribute(
       ui::AX_ATTR_NAME,
@@ skipping 19 matching lines @@
   // If a widget changes visibility it may affect what's focused, in particular
   // when a widget that contains the focused view gets hidden.
   AXAuraObjCache::GetInstance()->OnFocusedViewChanged();
 }
 
 void AXWidgetObjWrapper::OnWillRemoveView(Widget* widget, View* view) {
   AXAuraObjCache::GetInstance()->RemoveViewSubtree(view);
 }
 
 }  // namespace views