how to custom checkers for iOS

Hi, I use infer in my Objective-C project, The result is not accuracy，for example,I use method check nonull :
` 13. if (XMUtils.validateString(A)) {
14. > [components addObject:A];
15. }` ,
But result show NULL_DEREFERENCE ，I want to custom checkers for filter this problems, how to do I will ？[fbinfer.com](https://fbinfer.com/) is not detailing, I dont find more detailed information。