#vulnerable code 
                              @Override
  public String toString() {
    StringBuilder builder = new StringBuilder();
    builder.append("ServiceFlow [\r\n\tflowName = ");
    builder.append(flowName);
    builder.append("\r\n\t");
    Set<ServiceConfig> nextServices = servicesOfFlow.get(getHeadServiceConfig().getServiceName());

    builder.append(getHeadServiceConfig().getSimpleDesc());
    builder.append(" --> ");
    getHeadServiceConfig().getNextServiceConfigs().forEach(item -> {
      builder.append(item.getSimpleDesc()).append(",");
    });

    if (nextServices != null) {
      for (Map.Entry<String, Set<ServiceConfig>> entry : servicesOfFlow.entrySet()) {
        if (getHeadServiceConfig().getServiceName().equals(entry.getKey())) {
          continue;
        }
        builder.append("\r\n\t");
        builder.append(getServiceConfig(entry.getKey()).getSimpleDesc());
        builder.append(" -- > ");
        entry.getValue().forEach(item -> {
          builder.append(item.getSimpleDesc()).append(", ");
        });
      }
    }


    builder.append("\n]");
    return builder.toString();
  } 
                            #location 11 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              @Override
  public String toString() {
    StringBuilder builder = new StringBuilder();
    builder.append("ServiceFlow [ flowName = ");
    builder.append(flowName);
    builder.append("\r\n\t");

    ServiceConfig hh = header;
    buildString(hh, builder);
    builder.append("\n]");
    return builder.toString();
  }