setting model attriputes

Is it possible to set a model attribute from an ajax call to a FragmentController method that has a FragmentModel or PageModel as a parameter?

My ajax call is made when a user on the fragment list is clicked

                type: "GET",
                url: '${ui.actionLink( "openmrscustom", "admin/account", "getUserAccount" )}',
                global: false,
                async: false,
                data: {
                    personId: personId,
                success: function (data) {

My method on the controller

  public String getUserAccount(PageModel model, @RequestParam(value = "personId") Integer personId) {

                System.out.println(new Account(Context.getPersonService().getPerson(personId)).toString());
                model.addAttribute("account", "Account");  
                return null;

the problem I have is the model.addAttribute throws a null pointer

@dkayiwa @miirochristopher