MVC 5在控制器中传递Null对象错误

Question

这是我的代码，这里发现了这个错误＆＃34; System.NullReferenceException：＆＃39;对象引用未设置为对象的实例。＆＃39;＆＃34;这是运行时错误，_DefaultRepo显示为NULL。

public class DefaultController : Controller
    {
        // GET: Default
        private readonly IDefautlRepository _DefaultRepo;

        private readonly LoginRepository _Login;

        public DefaultController()
        {

        }

        public DefaultController(IDefautlRepository DefaultRepo, LoginRepository Login)
        {
            this._DefaultRepo = DefaultRepo;
            this._Login = Login;
        }

        public ActionResult Index()
        {
            return View();
        }

        public JsonResult GetGroupId (string EmailId , string Password)
        {
            try
            {
                string _emailId;
                string _password;
                MT_UserDetails User= new MT_UserDetails();                
                _emailId = EmailId;
                _password = Password;

                User = this._DefaultRepo.GetGroupId(_emailId, _password);              
                Session["GroupId"] = User.GroupId;
            }
            catch (Exception e)
            {
                throw e;
            }
            return Json(false, JsonRequestBehavior.AllowGet);
        }
    }
}