how to solve these error in mvc grid??

my code is

model//

public class ContactDetails
{
public int ID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string Country { get; set; }
public DateTime DOB { get; set; }
public string ContactNo { get; set; }
public bool IsActive { get; set; }
}

controller///

public class ExampleController : Controller
{
// GET: Example
public ActionResult Example()
{
if (Session["model"] == null)
Session["model"] = GetContactDetails();
var model = (List)Session["model"];
return View(model);
}
public JsonResult SaveContact(ContactDetails model)
{
var modelList = (List)Session["model"];
if (model.ID > 0)
{
var index = modelList.FindIndex(x => x.ID == model.ID);
modelList[index] = model;
}
else
{
var item = modelList.LastOrDefault();
model.ID = item.ID + 1;
modelList.Add(model);
}
Session["model"] = modelList;
return Json(new { ID = model.ID, FirstName = model.FirstName, LastName = model.LastName, Country = model.Country, ContactNo = model.ContactNo, DOB = model.DOB, IsActive = model.IsActive });
}
public JsonResult DeleteContact(ContactDetails model)
{
var modelList = (List)Session["model"];
var item = modelList.Find(x => x.ID == model.ID);
modelList.Remove(item);
Session["model"] = modelList;
return Json(new { ID = model.ID });
}

List GetContactDetails()
{
List contact = new List{
new ContactDetails{ID=1,FirstName="Doe", LastName="John", Country="United State", ContactNo="4445454590", DOB=new DateTime(1988,3,23), IsActive=true},
new ContactDetails{ID=2,FirstName="Chang", LastName="Lee", Country="China", ContactNo="3323334433", DOB=new DateTime(1970,5,3), IsActive=true},
new ContactDetails{ID=3,FirstName="Ajay", LastName="Kumar", Country="India", ContactNo="9723334444", DOB=new DateTime(1990,6,2), IsActive=true},
new ContactDetails{ID=4,FirstName="Merry", LastName="John", Country="United State", ContactNo="2332456678", DOB=new DateTime(1978,1,1), IsActive=true},
new ContactDetails{ID=5,FirstName="Kiran", LastName="Kher", Country="India", ContactNo="9999944566", DOB=new DateTime(1998,10,29), IsActive=true}
};
return contact;
}
}

view///

@model ICollection
@{
ViewBag.title = "WebGrid";
}