在矩阵表MVC ASP.NET

Question

您好，我在调试时有一个空引用异常，这是我的代码

 public class MatriceViewModel
        {
            public string DateDebForm { get; set; }
            public long ID_PNT { get; set; }
            public string NomPn { get; set; }
            public string TypeForm { get; set; }

            public string DateFinForm { get; set; }
        }
        public ActionResult Matrice()
        {
            List<MatriceViewModel> mylist = new List<MatriceViewModel>();

            ViewBag.forma = (from s in db.GetListAllStatDT().ToList() select s.TypeQuestionExam).Distinct();

            mylist = db.GetListAllStatDT().Select(c => new MatriceViewModel
            {
                DateDebForm = c.DateDebutExam,
                ID_PNT = c.ID_PNT ?? 0,
                NomPn = c.Nom,
                TypeForm = c.TypeQuestionExam,
                DateFinForm = c.DateExpirer
            }).ToList();

            List<System.Linq.IGrouping<string, MatriceViewModel>> model = (from a in mylist group a by a.NomPn into g select g).ToList();

            return View(model);
        }

@model  IEnumerable<System.Linq.IGrouping<string, BSA_TAL.Controllers.HomeController.MatriceViewModel>>
 <table align="center">
                    @{  IEnumerable<string> bList = ViewBag.forma; }
                    <tr>
                        <td>Nom/Formation</td>
                                                @for (int i = 0; i < bList.Count(); i++)
                                                {
                                                    <td></td>
                                                    <td class="alert-info" style="text-align:center">@bList.ElementAt(i)</td>
                        }

                    </tr>
                    @foreach (var a in Model)
            {

                <tr>

                    <td class="alert-info" style="text-align:center">@a.Key</td>

                    @for (int i = 0; i < bList.Count(); i++)
                    {

                          <td>@a.FirstOrDefault(b => b.NomPn == bList.ElementAt(i)).DateDebForm</td>


                    }
                    }





                        </tr>
                    }
                </table>

空引用位于“ @ a.FirstOrDefault（b => b.NomPn == bList.ElementAt（i））。DateDebForm”

我以相同的方法使用了另一个带有静态值和参数的矩阵，它起作用了，但是当我使用数据库时，它向我显示了空引用

对不起，我的英语不好

感谢您的帮助