Error in the Edit

Question

Hi Everybody,

I have and mvc3 application:

ActivityController:

[code]
public ActionResult Edit(int id)
        {
            Activity activity = db.Activities.Find(id);
            PopulateProjectsDropDownList(activity.ProjectID);

            activity = db.Activities
               .Include(i => i.Employee)
               .Where(i => i.ActivityID == id)
               .Single();
            PopulateAssignedEmployeeData(activity);

          
            return View(activity);
        }//end method


        [HttpPost]
        public ActionResult Edit(int? id, FormCollection formCollection, string[] selectedEmployees)
        {
            if (id == null)
            {
                return View("Index");
            }

            var actvityToUpdate = db.Activities
                .Include(a => a.Employee)
                .Where(i => i.ActivityID == id)
                .Single();
            if (TryUpdateModel(actvityToUpdate, "", null, new string[]{"Employees"} ))
            {
                try
                {
                    UpdateActivityEmployeeData(selectedEmployees, actvityToUpdate);
                    db.Entry(actvityToUpdate).State = EntityState.Modified;
                    db.SaveChanges();
                    return Redirect("Index");
                    //if (string.IsNullOrWhiteSpace(actvityToUpdate.))
                    //{
                       
                    //}
                }
                catch (Exception exception)
                {
                   
                   //Log the error (add a variable name after DataException)
            ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists save urself");
                    ModelState.AddModelError("",exception.Message);
                   // ModelState.AddModelError("",form );
                }//end catch
            }
            PopulateAssignedEmployeeData(actvityToUpdate);
            return View(actvityToUpdate);

        }//end method
[/code]

Model Activity:

[code]
public class Activity
    {
        //[Key]
        //[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        //[Display(Name = "Nummer(willekeurig nummer)")]
        //[Key]
        public int ActivityID { get; set; }
        [Required(ErrorMessage = "Naam moet wel ingevuld worden")]
        [MaxLength(50)]
        [Display(Name = "Activiteit")]
        public string Name { get; set; }

        [Display(Name = "Omschrijving")]
        public string Description { get; set; }

        [Display(Name = "uren")]
        public decimal Hour { get; set; }
       
        [DataType(DataType.Date)]
        [Display(Name = "Begin datum")]
        public DateTime BeginDate { get; set; }       
       
        [DataType(DataType.Date)]
        [Display(Name = "Eind datum")]
        public DateTime EndDate { get; set; }

        [Display(Name = "Tarief")]
        public decimal Tariff { get; set; }

        [Display(Name = "Actief?")]
        public bool Active { get; set; }
       
        [Display(Name = "project")]
        public int ProjectID { get; set; }
        //public virtual DbSet<Project> Project { get; set; }
        public virtual Project Project { get; set; }
        public virtual ICollection<Employee> Employee { get; set; }
        //public virtual ICollection<Activity> Activities { get; set; }
        public virtual ICollection<Hour> Hours { get; set; }

    }
[/code]

But when I try to Edit the form, for example this Object:
http://localhost:56610/Activity/Edit/4

And I post, then I get this error:

Server Error in '/' Application.

---

The parameters dictionary contains a null entry for parameter 'id' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult Edit(Int32)' in 'ManyToMany.Controllers.ActivityController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.
Parameter name: parameters

and the routing is then:

http://localhost:56610/Activity/Edit/Index

THX