code: def get_existing_item_id(self, api):
          
          resp = api.get_existing_item(self)
          items = resp.json()
          num_items = len(items)
          if num_items == 0:
              return None
          else:
              return items[0]['id']
docs: Lookup the id for this item via the D4S2 service.
          :param api: D4S2Api object who communicates with D4S2 server.
          :return str id of this item or None if not found