docs: @Override
      public void addNotificationHandler(String serviceName, NotificationHandler handler) throws ServiceException {
  
          ServiceInstanceUtils.validateManagerIsStarted(isStarted.get());
          ServiceInstanceUtils.validateServiceName(serviceName);
          if (handler == null) {
              throw new ServiceException(ErrorCode.SERVICE_DIRECTORY_NULL_ARGUMENT_ERROR,
                      ErrorCode.SERVICE_DIRECTORY_NULL_ARGUMENT_ERROR.getMessageTemplate(),
                      "NotificationHandler");
          }
          ModelService service = getLookupService().getModelService(serviceName);
          if (service == null) {
              throw new ServiceException(ErrorCode.SERVICE_DOES_NOT_EXIST,ErrorCode.SERVICE_DOES_NOT_EXIST.getMessageTemplate(),serviceName);
          }
          getLookupService().addNotificationHandler(serviceName, handler);
  
      }