Fix crash on clicking the web notification tray

ui/message_center/views/message_center_bubble.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 "ui/message_center/views/message_center_bubble.h"
 
 #include "base/macros.h"
 #include "ui/base/l10n/l10n_util.h"
 #include "ui/message_center/message_center_style.h"
 #include "ui/message_center/views/message_center_view.h"
@@ skipping 89 matching lines @@
   message_center_view_ = new MessageCenterView(
       message_center(), tray(), max_height(), initially_settings_visible_,
       false); /* MessageCenterBubble should be used only on ChromeOS.
                  Message center is never shown top down in ChromeOS. */
   bubble_view()->AddChildView(new ContentsView(this, message_center_view_));
   // Resize the content of the bubble view to the given bubble size. This is
   // necessary in case of the bubble border forcing a bigger size then the
   // |new_bubble_view| actually wants. See crbug.com/169390.
   bubble_view()->Layout();
   UpdateBubbleView();
-  bubble_view()
-      ->GetFocusManager()
-      ->GetNextFocusableView(nullptr, nullptr, false, false)
-      ->RequestFocus();
+  views::FocusManager* focus_manager = bubble_view()->GetFocusManager();
+  // new_bubble_view should be a top level view and have a focus manager.
+  DCHECK(focus_manager);
+  views::View* next_focusable_view = focus_manager
+      ->GetNextFocusableView(nullptr, nullptr, false, false);
+  // The bubble may not have any focusable view (eg. on lock screen). In such
+  // case, |next_focusable_view| is null.
+  if (next_focusable_view)
+    next_focusable_view->RequestFocus();
 }
 
 void MessageCenterBubble::OnBubbleViewDestroyed() {
   message_center_view_ = NULL;
 }
 
 void MessageCenterBubble::UpdateBubbleView() {
   if (!bubble_view())
     return;  // Could get called after view is closed
   const NotificationList::Notifications& notifications =
@@ skipping 14 matching lines @@
 }
 
 void MessageCenterBubble::OnMouseExitedView() {
 }
 
 size_t MessageCenterBubble::NumMessageViewsForTest() const {
   return message_center_view_->NumMessageViewsForTest();
 }
 
 }  // namespace message_center