MAGETWO-88116: [Backport-2.2] of PR-#10935 Fix LowStock report in All Websites view #13682

Author: ishakhsuvarov

app/code/Magento/Reports/Block/Adminhtml/Product/Lowstock/Grid.php

@@ -53,14 +53,12 @@ protected function _prepareCollection()
         } elseif ($store) {
             $storeId = (int)$store;
         } else {
-            $storeId = '';
+            $storeId = null;
         }
 
         /** @var $collection \Magento\Reports\Model\ResourceModel\Product\Lowstock\Collection  */
         $collection = $this->_lowstocksFactory->create()->addAttributeToSelect(
             '*'
-        )->setStoreId(
-            $storeId
         )->filterByIsQtyProductTypes()->joinInventoryItem(
             'qty'
         )->useManageStockFilter(