code: private static String checkUserIdentifier(final FormItemList formItemList,
  			final CreateUserResponse createUserResponse) {
  		final String userId = formItemList
  				.getField(ProtocolConstants.Parameters.Create.User.USER_IDENTIFIER);
  		if (userId != null) {
  			final Node user = NeoUtils.getUserByIdentifier(userId);
  			if (user == null) {
  				return userId;
  			}
  
  			createUserResponse.userIdentifierInvalid(userId);
  		} else {
  			createUserResponse.userIdentifierMissing();
  		}
  
  		return null;
  	}
docs: check if the request contains a valid user identifier
  
  @param formItemList
  form item list extracted
  @param createUserResponse
  response object
  @return user identifier<br>
  <b>null</b> if the identifier is invalid