Object reference error trying to use RenderPartial - asp.net-mvc-3

Question

I have a main view using a ViewModel. Inside the ViewModel I do this (Edited to show complete ViewModel):
namespace MyNameSpace.ViewModels
{
public class MyViewModel
{
public ModelOne ModelOne { get; set; }
public ModelTwo ModelTwo { get; set; }
}
}
On my main view I do this (EDIT: Added #Html.Hidden code):
#using MyNameSpace.ViewModels
#using MyNameSpace.Models
#model MyViewModel
...
#using (Html.BeginFormAntiForgeryPost())
{
#Html.Hidden("myData", new MvcSerializer()
.Serialize(Model, SerializationMode.Signed))
....
<div>
#{Html.RenderPartial("_MyCheckBox",
Model.ModelTwo, new ViewDataDictionary());}
</div>
}
....
My partial view is:
#using MyNameSpace.Models
#model ModelTwo
<div>
<fieldset>
<div class="editor-label">
#Html.LabelFor(x => x.MyCheckBox)</div>
<div class="editor-field">
<select multiple="multiple" id="#Html.FieldIdFor(x =>
x.MyCheckBox)" name="#Html.FieldNameFor(x =>
x.MyCheckBox)">
#foreach (MyEnum item in Enum.GetValues(typeof(MyEnum)))
{
var selected = Model.MyCheckBox.Contains(item); //ERROR HERE
<option value="#((int)item)" #if (selected)
{<text>selected="selected"</text>}>
#T(item.ToString())
</option>
}
</select>
</div>
</fieldset>
</div>
I am getting the Object reference not set to an instance ... error and am not sure how to correct it.
Originally, I had that <fieldset> inside my main view and was getting that error. I thought it was because of the two models and that's why I placed it in a partial view. But only to discover I am running into the same problem.
I am not setting the MyCheckBox in my partial view on the line var selected = Model.MyCheckBox.Contains(item); properly.
Any thoughts?
EDIT (Adding MyCheckBox code)
Here is MyCheckBox inside ModelOne.cs:
[Mandatory(ErrorMessage = "Please select at least one option")]
[Display(Name = "Please select one ore more options")]
[MySelector]
public virtual int MyCheckBox { get; set; }
And here it is inside ModelTwo.cs:
private IList<MyEnum> _myEnum;
public IList<MyEnum> MyCheckBox
{
get
{
if (_myEnum== null)
{
_myEnum= new List<MyEnum>();
foreach (MyEnumitem in Enum.GetValues(typeof(MyEnum)))
{
if (((MyEnum)Record.MyCheckBox& item) == item)
_myEnum.Add(item);
}
}
return _myEnum;
}
set
{
_myEnum= value;
Record.MyCheckBox= 0;
foreach (var item in value)
{
Record.MyCheckBox|= (int)item;
}
}
}
Please note, I am using Orchard (hence the Record) which, in turn, uses NHibernate. I don't believe that is relevant, but I could be wrong. The MyCheckBox code is using [Flags] attribute of enum and storing the value as an int in the DB (hence the proxy). Here is what the enum would look like:
MyEnum.cs:
[Flags]
public enum MyEnum
{
[Display(Name="Name 1")]
Enum1 = 1,
[Display(Name="Name 2")]
Enum2 = 2,
[Display(Name="Name 3")]
Enum3 = 4,
[Display(Name="Name 4")]
Enum4 = 8,
[Display(Name="Name 5")]
Enum5 = 16
}
MyController.cs
private MyViewModel myData;
protected override void OnActionExecuting(ActionExecutingContext filterContext)
{
var serialized = Request.Form["myData"];
if (serialized != null) //Form was posted containing serialized data
{
myData= (MyViewModel)new MvcSerializer().Deserialize
(serialized, SerializationMode.Signed);
TryUpdateModel(myData);
}
else
myData= (MyViewModel)TempData["myData"] ?? new MyViewModel();
TempData.Keep();
}
protected override void OnResultExecuted(ResultExecutedContext filterContext)
{
if (filterContext.Result is RedirectToRouteResult)
TempData["myData"] = myData;
}
Then in the particular Action within *MyController.cs:
public ActionResult Step5(string backButton, string nextButton)
{
if (backButton != null)
return RedirectToAction("Step4");
else if ((nextButton != null) && ModelState.IsValid)
return RedirectToAction("Confirm");
else
return View(myData);
}