500에러, findById(null)

// 거래 체결
	@Transactional
	public Contract save(ContractAddDTO contractDto) {
		Contract contract = new Contract();
		// product 객체 설정
		Product product = productRepository.findById(contractDto.getProductId())
				.orElseThrow(()->new IllegalArgumentException("상품 아이디를 찾을 수 없습니다."));
		contract.setProduct(product);
		
	
		// buying 객체 설정, nullable
		Optional<Buying> buying = buyingRepository.findById(contractDto.getBuyingId());
		if(buying.isPresent()) {
			contract.setBuying(buying.get());	
		} else {
			contract.setBuying(null);	
		}
		// selling 객체 설정, nullable
		Optional<Selling> selling = sellingRepository.findById(contractDto.getSellingId());
		if(selling.isPresent()) {
			contract.setSelling(selling.get());		
		} else {
			contract.setSelling(null);
		}
		// 나머지
		contract.setBuyerNumber(contractDto.getBuyerNumber());
		contract.setSellerNumber(contractDto.getSellerNumber());
		contract.setContractDate(LocalDateTime.now());
		contract.setSellingStatus(contractDto.getSellingStatus());
		contract.setBuyingStatus(contractDto.getBuyingStatus());
		contract.setPrice(contractDto.getPrice());		
		contract.setRegistDate(LocalDateTime.now());		
		
		return contractRepository.save(contract);
	}

optional 설정을 했으니 null값이 들어와도 문제 없는줄 알았음.
but findById(null)을 하게 되니 거기서 그냥 멈춰버린듯...

	if(contractDto.getBuyingId() != null) {
			Optional<Buying> buying = buyingRepository.findById(contractDto.getBuyingId());
			if(buying.isPresent()) {
				contract.setBuying(buying.get());	
			} else {
				contract.setBuying(null);	
			}
		}
		
		if(contractDto.getSellingId() != null) {
			Optional<Selling> selling = sellingRepository.findById(contractDto.getSellingId());
			if(selling.isPresent()) {
				contract.setSelling(selling.get());		
			} else {
				contract.setSelling(null);
			}
		}

이렇게 수정하고 해결됨