AppListModel / AppsGridView: Better error handling for corner cases.

ui/app_list/views/apps_grid_view.cc

Index: ui/app_list/views/apps_grid_view.cc
diff --git a/ui/app_list/views/apps_grid_view.cc b/ui/app_list/views/apps_grid_view.cc
index 5df27a63e79c8a455c62956c478f65b4ea82a8da..140ae4a2a75db94cdf2dfe839d2077e7150321aa 100644
--- a/ui/app_list/views/apps_grid_view.cc
+++ b/ui/app_list/views/apps_grid_view.cc
@@ -1737,8 +1737,14 @@ void AppsGridView::MoveItemToFolder(AppListItemView* item_view,
   const std::string& source_item_id = item_view->item()->id();
   AppListItemView* target_view =
       GetViewDisplayedAtSlotOnCurrentPage(target.slot);
+  DCHECK(target_view);
   const std::string& target_view_item_id = target_view->item()->id();
 
+  // Check that the item is not being dropped onto itself; this should not
+  // happen, but it can if something allows multiple views to share an
+  // item (e.g., if a folder drop does not clean up properly).
+  DCHECK_NE(source_item_id, target_view_item_id);
+
   // Make change to data model.
   item_list_->RemoveObserver(this);
   std::string folder_item_id =