Unit test of an action ends in an error

I have an Edit Post action method in my MVC4 application and I am trying to unit test this action. But, the Unit test fails with "NullReferenceException". Below is the unit test FYR.

    [TestMethod]
    public void EditAction_Should_Redirect_When_Update_Successful()
    {
        // Arrange

        var mockHttpContext = new Mock<HttpContextBase>();            
        var mockRequest = new Mock<HttpRequestBase>();
        mockHttpContext.Setup(x => x.Request).Returns(mockRequest.Object);


        // tell the mock to return "POST" when HttpMethod is called      
        mockRequest.Setup(x => x.HttpMethod).Returns("POST");

        mockRequest.SetupGet(req => req.Form).Returns(new FormCollection());

        var controller = GetTheController();
        var id = 1;

        // assign the fake context
        var context = new ControllerContext(mockHttpContext.Object, 
              new RouteData(), 
              controller);
        controller.ControllerContext = context;


        var formValues = new FormCollection() { 
             {"MyModel.Id", "1" },
             {"MyModel.ActivityDescription", "This is another description"},
             {"MyModel.CreatedDate", "31-12-2014"},
             {"MyModel.UserId", "1"},
             {"MyModel.IsCompleted", "false"}
        };                                       

        // Act
        var result = controller.Edit(id, formValues) as RedirectToRouteResult;

        // Assert
        Assert.AreEqual("List", result.RouteValues["Action"]);
        Assert.AreEqual(id, result.RouteValues["id"]);
    }

Edit action method is below -

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Edit(int id, FormCollection collection)
    {
        var item = _itemsRepository.Find(id);
        var viewResult = ValidateItem(item);
        if (viewResult != null)
            return viewResult;

        //Unit test is failing at this step.
        TryUpdateModel(item);
        if (ModelState.IsValid)
        {
            _itemsRepository.Edit(item);
            return RedirectToAction("Index");
        }
        else return View(item);
    }

Below is the stacktrace for reference -

Test Outcome: Failed Test Duration: 0:00:00.3306816

Result Message:

Test method MvcToDoListItemsDemo.Tests.TodoControllerTest.EditAction_Should_Redirect_When_Update_Successful threw exception: 
System.NullReferenceException: Object reference not set to an instance of an object.
Result StackTrace:  
at Microsoft.Web.Infrastructure.DynamicValidationHelper.DynamicValidationShim.IsValidationEnabled(HttpContext context)
   at Microsoft.Web.Infrastructure.DynamicValidationHelper.ValidationUtility.IsValidationEnabled(HttpContext context)
   at Microsoft.Web.Infrastructure.DynamicValidationHelper.ValidationUtility.GetUnvalidatedCollections(HttpContext context, Func`1& formGetter, Func`1& queryStringGetter)
   at System.Web.Helpers.Validation.Unvalidated(HttpRequest request)
   at System.Web.Mvc.FormValueProviderFactory.<.ctor>b__0(ControllerContext cc)
   at System.Web.Mvc.FormValueProviderFactory.GetValueProvider(ControllerContext controllerContext)
   at System.Web.Mvc.ValueProviderFactoryCollection.<>c__DisplayClassc.<GetValueProvider>b__7(ValueProviderFactory factory)
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at System.Web.Mvc.ValueProviderFactoryCollection.GetValueProvider(ControllerContext controllerContext)
   at System.Web.Mvc.ControllerBase.get_ValueProvider()
   at System.Web.Mvc.Controller.TryUpdateModel[TModel](TModel model)

Could someone please advise if I am doing anything wrong here ?

Regards, Ram