output: #fixed code
                                  @Override
      public ValidateResult validate(AliasTransaction tx) {
          Alias alias = tx.getTxData();
          if (StringUtils.isBlank(alias.getAddress()) || new Address(alias.getAddress()).getHash().length != 23) {
              return ValidateResult.getFailedResult("The address format error");
          }
          if (!StringUtils.validAlias(alias.getAlias())) {
              return ValidateResult.getFailedResult("The alias is between 3 to 20 characters");
          }
  
          List<Transaction> txList = getLedgerService().getCacheTxList(TransactionConstant.TX_TYPE_SET_ALIAS);
          if (txList != null && tx.size() > 0) {
              for (Transaction trx : txList) {
                  Alias a = ((AliasTransaction) trx).getTxData();
                  if(alias.getAddress().equals(a.getAlias())) {
                      return ValidateResult.getFailedResult("The alias has been occupied");
                  }
                  if(alias.getAlias().equals(a.getAlias())){
                      return ValidateResult.getFailedResult("The alias has been occupied");
                  }
              }
          }
  
          AliasPo aliasPo = getAliasDataService().get(alias.getAlias());
          if (aliasPo != null) {
              return ValidateResult.getFailedResult("The alias has been occupied");
          }
          return ValidateResult.getSuccessResult();
      }
input: #vulnerable code 
                                  @Override
      public ValidateResult validate(AliasTransaction tx) {
          Alias alias = tx.getTxData();
          if (StringUtils.isBlank(alias.getAddress()) || new Address(alias.getAddress()).getHash().length != 23) {
              return ValidateResult.getFailedResult("The address format error");
          }
          if (!StringUtils.validAlias(alias.getAlias())) {
              return ValidateResult.getFailedResult("The alias is between 3 to 20 characters");
          }
  
          AliasPo aliasPo = getAliasDataService().get(alias.getAlias());
          if (aliasPo != null) {
              return ValidateResult.getFailedResult("The alias has been occupied");
          }
          return ValidateResult.getSuccessResult();
      } 
                              #location 11 
                              #vulnerability type NULL_DEREFERENCE