Fix crash in screensaver shutdown.

chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.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 "chrome/browser/chromeos/kiosk_mode/kiosk_mode_screensaver.h"
 
 #include "ash/screensaver/screensaver_view.h"
 #include "ash/shell.h"
 #include "ash/wm/user_activity_detector.h"
 #include "base/bind.h"
@@ skipping 109 matching lines @@
     // Delete it.
     content::BrowserThread::PostTask(
         content::BrowserThread::FILE,
         FROM_HERE,
         base::Bind(
             &extension_file_util::DeleteFile, extension_base_path_, true));
   }
 
   // In case we're shutting down without ever triggering the active
   // notification and/or logging in.
-  ash::Shell::GetInstance()->user_activity_detector()->RemoveObserver(this);
+  if (ash::Shell::GetInstance() &&
+      ash::Shell::GetInstance()->user_activity_detector())
+    ash::Shell::GetInstance()->user_activity_detector()->RemoveObserver(this);
   registrar_.RemoveAll();
 }
 
 void KioskModeScreensaver::GetScreensaverCrxPath() {
   chromeos::KioskModeSettings::Get()->GetScreensaverPath(
       base::Bind(&KioskModeScreensaver::ScreensaverPathCallback,
                  weak_ptr_factory_.GetWeakPtr()));
 }
 
 void KioskModeScreensaver::ScreensaverPathCallback(
@@ skipping 88 matching lines @@
 
   g_kiosk_mode_screensaver = new KioskModeScreensaver();
 }
 
 void ShutdownKioskModeScreensaver() {
   delete g_kiosk_mode_screensaver;
   g_kiosk_mode_screensaver = NULL;
 }
 
 }  // namespace chromeos