aura: Fix a crash when moving mouse over the empty space on the root window.

ui/aura/root_window.cc

 // Copyright (c) 2011 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/aura/root_window.h"
 
 #include "base/logging.h"
 #include "base/utf_string_conversions.h"
 #include "ui/aura/event.h"
 #include "ui/aura/focus_manager.h"
 #include "ui/aura/window_delegate.h"
 #include "ui/base/events.h"
 
 namespace aura {
 namespace internal {
 
 RootWindow::RootWindow()
     : Window(NULL),
       mouse_pressed_handler_(NULL),
       ALLOW_THIS_IN_INITIALIZER_LIST(focus_manager_(new FocusManager(this))) {
   set_name(ASCIIToUTF16("RootWindow"));
 }
 
 RootWindow::~RootWindow() {
 }
 
 bool RootWindow::HandleMouseEvent(const MouseEvent& event) {
   Window* target = mouse_pressed_handler_;
   if (!target)
     target = GetEventHandlerForPoint(event.location());

[sadrul, 2011/09/21 14:24:39]

Or may be return early if target is NULL here?

   if (event.type() == ui::ET_MOUSE_PRESSED && !mouse_pressed_handler_)
     mouse_pressed_handler_ = target;
   if (event.type() == ui::ET_MOUSE_RELEASED)
     mouse_pressed_handler_ = NULL;
-  if (target->delegate()) {
+  if (target && target->delegate()) {
     MouseEvent translated_event(event, this, target);
     return target->OnMouseEvent(&translated_event);
   }
   return false;
 }
 
 bool RootWindow::HandleKeyEvent(const KeyEvent& event) {
   Window* focused_window = GetFocusManager()->focused_window();
   if (focused_window) {
     KeyEvent translated_event(event);
     return GetFocusManager()->focused_window()->OnKeyEvent(&translated_event);
   }
   return false;
 }
 
 FocusManager* RootWindow::GetFocusManager() {
   return focus_manager_.get();
 }
 
 }  // namespace internal
 }  // namespace aura