Fix for BomItem clean function
Fix for BomItem clean function

Handle the case where the sub_part does not exist

SchrodingersGat/InvenTree,SchrodingersGat/InvenTree,SchrodingersGat/InvenTree,inventree/InvenTree,inventree/InvenTree,inventree/InvenTree,inventree/InvenTree,SchrodingersGat/InvenTree