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