Fix crash in ExtensionUninstallDialogView. It needs to verify Shell

chrome/browser/ui/views/extensions/extension_uninstall_dialog_view.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/extensions/extension_uninstall_dialog.h"
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/string_util.h"
 #include "base/utf_string_conversions.h"
@@ skipping 23 matching lines @@
 class ExtensionUninstallDialogDelegateView;
 
 // Returns parent window for extension uninstall dialog.
 // For ash, use app list window if it is visible.
 // For other platforms or when app list is not visible on ash,
 // use the given browser window.
 // Note this function could return NULL if ash app list is not visible and
 // there is no browser window.
 gfx::NativeWindow GetParent(Browser* browser) {
 #if defined(USE_ASH)
-  gfx::NativeWindow app_list = ash::Shell::GetInstance()->GetAppListWindow();
-  if (app_list)
-    return app_list;
+  if (ash::Shell::HasInstance()) {
+    gfx::NativeWindow app_list = ash::Shell::GetInstance()->GetAppListWindow();
+    if (app_list)
+      return app_list;
+  }
 #endif
 
   if (browser && browser->window())
     return browser->window()->GetNativeWindow();
 
   return NULL;
 }
 
 // Views implementation of the uninstall dialog.
 class ExtensionUninstallDialogViews : public ExtensionUninstallDialog {
@@ skipping 184 matching lines @@
 
 }  // namespace
 
 // static
 ExtensionUninstallDialog* ExtensionUninstallDialog::Create(
     Profile* profile,
     Browser* browser,
     Delegate* delegate) {
   return new ExtensionUninstallDialogViews(profile, browser, delegate);
 }