app_list: Fix sync animation crash.

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 d8f92c305ada4b7812851763baae3357477a32f2..f300de7fdd22400d750c8f6a4fe52b28ce640eae 100644
--- a/ui/app_list/views/apps_grid_view.cc
+++ b/ui/app_list/views/apps_grid_view.cc
@@ -657,8 +657,9 @@ void AppsGridView::UpdatePaging() {
 }
 
 void AppsGridView::UpdatePulsingBlockViews() {
+  const int existing_items = item_list_ ? item_list_->item_count() : 0;
   const int available_slots =
-      tiles_per_page() - item_list_->item_count() % tiles_per_page();
+      tiles_per_page() - existing_items % tiles_per_page();
   const int desired = model_->status() == AppListModel::STATUS_SYNCING ?
       available_slots : 0;