add inventory to frontend

backend/models/InventoryItemSearch.php

@@ -71,7 +71,7 @@ class InventoryItemSearch extends InventoryItem
     			'inventory.created_at as inventory_created_at',
     			'inventory_prev.id_inventory as inventory_prev_id_inventory',
     			'inventory_prev.name as inventory_prev_name',
-    			new Expression('(inventory_item.count - ( inventory_item.count_prev + inventory_item.count_in - inventory_item.count_sold )) as item_difference'),
+    			new Expression('(coalesce(inventory_item.count,0) - ( coalesce(inventory_item.count_prev,0) + coalesce(inventory_item.count_in,0) - coalesce(inventory_item.count_sold,0) )) as item_difference'),
     	]);
     	
     	$query->from(InventoryItem::tableName());