fix: not able to transfer raw materials for subcontracted items (#18650)

diff --git a/erpnext/buying/doctype/purchase_order/purchase_order.py b/erpnext/buying/doctype/purchase_order/purchase_order.py
index 8117d9d..6ed6766 100644
--- a/erpnext/buying/doctype/purchase_order/purchase_order.py
+++ b/erpnext/buying/doctype/purchase_order/purchase_order.py
@@ -483,7 +483,7 @@
 							'from_warehouse': rm_item_data["warehouse"],
 							'stock_uom': rm_item_data["stock_uom"],
 							'main_item_code': rm_item_data["item_code"],
-							'allow_alternative_item': item_wh[rm_item_code].get('allow_alternative_item')
+							'allow_alternative_item': item_wh.get(rm_item_code, {}).get('allow_alternative_item')
 						}
 					}
 					stock_entry.add_to_stock_entry_detail(items_dict)