code: private function getUnitIdentifier(array $variation)
      {
          if (empty($variation['unit'])) {
              return null;
          }
  
          // Unit
          $unitIdentity = $this->identityService->findOneBy([
              'adapterIdentifier' => (string) $variation['unit']['unitId'],
              'adapterName' => PlentymarketsAdapter::NAME,
              'objectType' => Unit::TYPE,
          ]);
  
          if (null === $unitIdentity) {
              throw new NotFoundException('missing mapping for unit');
          }
  
          return $unitIdentity->getObjectIdentifier();
      }
docs: @param array $variation
  
  @return null|string