試了２天實在搞不定，上來跟前輩們求救一下～
一個DropDownList，一個按鈕，按更新POST回去Controller更新資料，但不懂為什麼DropDownList那邊會發生錯誤？？？
原始碼如下，錯誤報錯在View裡，不是在Controller裡
Controller

public ActionResult EditFinalPaper(int cId, int pId)
{
    List<SelectListItem> PresentationListItem = new List<SelectListItem>();
    PresentationListItem.Add(new SelectListItem { Text = "口頭發表(Oral)", Value = "1" });
    PresentationListItem.Add(new SelectListItem { Text = "海報展示發表(Poster)", Value = "2" });
    PresentationListItem.Add(new SelectListItem { Text = "口頭和海報發表皆可(Oral or Poster)", Value = "3" });
    ViewBag.PresentationList = PresentationListItem;

    Paper p = paperSrv.GetPaperInfo(pId);
    return View(p);
}

[HttpPost]
public ActionResult EditFinalPaper(Paper paper)
{
    if (ModelState.IsValid)
    {
        try
        {
            int PaperId = paperSrv.EditFinalPaper(paper);
            return RedirectToAction("UploadFinalPaperFileP", "Paper", new { cId = paper.ConferenceId, pId = PaperId });
        }
        catch (Exception e)
        {
            throw;
        }
    }
    return View(paper);
}

public int EditFinalPaper(Paper paper)
{
    paper.LastUpdate = DateTime.Now;
    db.SaveChanges();
    return paper.PaperId;
}

View

@using (Html.BeginForm())
{
    <div class="form-horizontal">
        @Html.HiddenFor(model => model.ConferenceId)
        @Html.HiddenFor(model => model.PaperId)
        <div class="form-group">
            <label for="Presentation" class="control-label col-md-2">論文發表方式(Presentation)</label>
            <div class="col-md-9">
                @Html.DropDownList("Presentation", (IEnumerable<SelectListItem>)ViewBag.PresentationList, "===請選擇===", new { @class = "form-control" })
            </div>
        </div>
        
        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="更新" class="btn btn-success" id="Update" onclick="if(confirm('確認更新？')) { return true; } else { return false; };" />
            </div>
        </div>
    </div>
}

錯誤訊息

遇到這種Error真的不知道該怎麼解呀～在View裡中斷查錯也沒有得到有用的資訊！
找google大神的常見問題我也都避開了，還是一樣會Error～
我想很大問題是出在ViewBag.PresentationList這個東西出錯，但就不知道錯在那裡？要如何修正
其它頁面也都是用ViewBag來傳值沒問題
目前沒有方向了，上來求救一下～