<script type="text/javascript">
        $(document).ready(function () {

            $("#Shareitem").click(function (e) {
                var serviceURL = '/Register/FirstAjax';
                $.ajax({
                    type: "POST",
                    url: serviceURL,
                    data: '123',
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: successFunc,
                    error: errorFunc
                });
                function successFunc(data, status) {
                    alert(data);
                }
                function errorFunc() {
                    alert('error');
                }
            });
        });
    </script>

[HttpPost]
        public ActionResult FirstAjax(string value)
        {
            string test = value.Trim();
            return Json("PROF.VALI", JsonRequestBehavior.AllowGet);
        }

string test = value.Trim();

An exception of type 'System.NullReferenceException' occurred in UserRegister.dll but was not handled in user code