Fix crash in ReparentItemToAnotherFolder() on Windows app list.

ui/app_list/views/apps_grid_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 "ui/app_list/views/apps_grid_view.h"
 
 #include <algorithm>
 #include <set>
 #include <string>
 
@@ skipping 1653 matching lines @@
 
   item_list_->AddObserver(this);
   model_->AddObserver(this);
   UpdatePaging();
 }
 
 void AppsGridView::ReparentItemToAnotherFolder(views::View* item_view,
                                                const Index& target) {
   DCHECK(IsDraggingForReparentInRootLevelGridView());
 
+  AppListItemView* target_view =
+      static_cast<AppListItemView*>(GetViewAtSlotOnCurrentPage(target.slot));
+  if (!target_view)
+    return;
+
   // Make change to data model.
   item_list_->RemoveObserver(this);
 
   AppListItem* reparent_item = static_cast<AppListItemView*>(item_view)->item();
   DCHECK(reparent_item->IsInFolder());
   const std::string source_folder_id = reparent_item->folder_id();
   AppListFolderItem* source_folder =
       static_cast<AppListFolderItem*>(item_list_->FindItem(source_folder_id));
 
   // Remove the source folder view if there is only 1 item in it, since the
   // source folder will be deleted after its only child item merged into the
   // target item.
   if (source_folder->ChildItemCount() == 1u)
     DeleteItemViewAtIndex(view_model_.GetIndexOfView(activated_item_view()));
 
-  AppListItemView* target_view =
-      static_cast<AppListItemView*>(GetViewAtSlotOnCurrentPage(target.slot));
   AppListItem* target_item = target_view->item();
 
   // Move item to the target folder.
   std::string target_id_after_merge =
       model_->MergeItems(target_item->id(), reparent_item->id());
   if (target_id_after_merge.empty()) {
     LOG(ERROR) << "Unable to reparent to item id: " << target_item->id();
     item_list_->AddObserver(this);
     return;
   }
@@ skipping 392 matching lines @@
 void AppsGridView::SetAsFolderDroppingTarget(const Index& target_index,
                                              bool is_target_folder) {
   AppListItemView* target_view =
       static_cast<AppListItemView*>(
           GetViewAtSlotOnCurrentPage(target_index.slot));
   if (target_view)
     target_view->SetAsAttemptedFolderTarget(is_target_folder);
 }
 
 }  // namespace app_list