Çözüldü NET Core MVC, " There is no ViewData item of type 'IEnumerable<SelectListItem>' that has the key 'CategoryId' " Hatası

 [HttpGet]
        public IActionResult Add()
        {
            CategoryManager cm = new CategoryManager(new EFCategoryRepo());
            List<SelectListItem> categorievalue = (from x in cm.GetList()
                                               select new SelectListItem
                                               {
                                                   Text = x.CategoryName,
                                                   Value = x.CategoryId.ToString()
                                               }).ToList();
            ViewBag.cm = categorievalue;
            return View();
        }

        [HttpPost]
        public IActionResult Add(Article a )
        {
            CategoryManager cm = new CategoryManager(new EFCategoryRepo());

            ArticleValidator av = new ArticleValidator();
            ValidationResult result = av.Validate(a);

            if (result.IsValid)
            {
                a.ArticleStatus = true;
                a.ArticleCreatedDate = Convert.ToDateTime(DateTime.Now.ToShortDateString());
                return RedirectToAction("Index", "Blog" );
            }
            else
            {
                foreach (var item in result.Errors)
                {
                    ModelState.AddModelError(item.PropertyName, item.ErrorMessage);
                }
            }
          
            return View();
        }


View Tarafı :

@model BlogEntity.Concrete.Article

@{
    ViewData["Title"] = "Add";
    Layout = "~/Areas/Admin/Views/Shared/_Layout.cshtml";
}

<br />

<h2>Yeni Blog Ekle</h2>
<br />


@using (Html.BeginForm("Add", "Blog", FormMethod.Post))
{
    <h4>Blog Adı</h4>
    @Html.TextBoxFor(x => x.ArticleTitle, new { [USER=193939]@Class[/USER] = "form-control" })
    @Html.ValidationMessageFor(x => x.ArticleTitle, "", new { [USER=193939]@Class[/USER] = "text-danger" })
    <br />
    <h4>Blog Açıklama</h4>
    @Html.TextAreaFor(x => x.ArticleContent, 7, 3, new { [USER=193939]@Class[/USER] = "form-control" })
    @Html.ValidationMessageFor(x => x.ArticleContent, "", new { [USER=193939]@Class[/USER] = "text-danger" })
    <br />
    <h4>Blog Kategorisi</h4>
    @Html.DropDownListFor(x => x.CategoryId, (List<SelectListItem>)ViewBag.cm, new { [USER=193939]@Class[/USER] = "form-control" })
    @Html.ValidationMessageFor(x => x.CategoryId,"", new { [USER=193939]@Class[/USER] = "text-danger" })
    <br />
    @*<h4>Blog Resmi</h4>
    @Html.TextBoxFor(x => x.ArticleImage, new { [USER=193939]@Class[/USER] = "form-control" })
    @Html.ValidationMessageFor(x => x.ArticleImage, "", new { [USER=193939]@Class[/USER] = "text-danger" })
    <br />*@

    <button class="btn btn-info">Blog Ekle</button>
    <input type="button" class="btn btn-danger" value="Geri Dön" onclick="location.href = '@Url.Action("Index","Blog")'" />


}