MVC Error: The model item passed into the dictionary is null

I'm just trying to build a view but I'm getting the following error:

System.InvalidOperationException: The model item passed into the dictionary is null, but this dictionary requires a non-null model item of type 'System.DateTime

Now, I know why this is coming up, the particular field in the database is null, however it is supposed to be, as this is something that is edited at a later date. Here is my code:

Action

public ActionResult View(Int64? Id)
    {

        ModelContainer ctn = new ModelContainer();
        var item = from t in ctn.Items where t.ItemID == Id select t;
        return View(Item.First());
    }

Main View

<%@ Page Title="" Language="C#" MasterPageFile="~/Views/Shared/Administrator.Master" Inherits="System.Web.Mvc.ViewPage<myApp.Data.Item>" %>

<asp:Content ID="Content1" ContentPlaceHolderID="TitleContent" runat="server">
    View
</asp:Content>

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">

    <% Html.RenderPartial("Details", Model); %>

</asp:Content>

<asp:Content ID="Content3" ContentPlaceHolderID="Header" runat="server">
<h1>Details - <%= Model.MainItem %></h1>
</asp:Content>

Partial View

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<myApp.Data.Item>" %>
<%@ Import Namespace="myApp.Supplier.Web.Extensions" %>

    <fieldset>

        <legend>Information</legend>

        <div class="fieldset">

            <%= Html.LabelFor(m => m.MainItem)%>
            <%= Html.DisplayFor(m => m.MainItem, "FormTextShort")%><br />
            <%= Html.LabelFor(m => m.Supplier.Name)%>
            <%= Html.DisplayFor(m => m.Supplier.Name, "FormTextShor