the program can potentially dereference a null pointer thereby raising a null exception fortify issue

I am getting fortify issue even after putting all NullPointer checks. below is the code. do i have to do something else to resolve this

 public void test(PreferenceDTO p)  {
        HttpSession session = req.getSession(false);
        if(p!= null && session != null) {
            p = session.getAttribute(p.getScreenName())
        }

        ResponseBuilder resp = Response.ok(ConvertToJSOn(p));

    }

Fortify error: the program can potentially deference a null pointer thereby raising a null exception