argument null exception was unhandled by user

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MVC

Controller code

public ActionResult GatePass( )//for Populate the values on GatePass View
{
Guid aId = new Guid();
AppointmentModel objAppointmentModel = new AppointmentModel();
objAppointmentModel = this._iAppointmentService.GetAppointmentByAckId(aId);
return View(objAppointmentModel);


}


[HttpPost]//For Populate the values on GatePass View based on Given acknowledgementId
public ActionResult GatePass(AppointmentModel appointmentModel)
{



List<validationresult> ValidationResultlist = this._iAppointmentService.GatePassValidation(appointmentModel);
if (ValidationResultlist != null && ValidationResultlist.Count() > 0)
{
// VisitorModel objvisitorModel = this._iAppointmentService.InitiateVisitor();
// appointmentModel.SecurityQuestionList = objvisitorModel.SecurityQuestionList;

ControllerExtensions.AddModelErrors(this, ValidationResultlist);
//return View("GatePass", appointmentModel);
return View(appointmentModel);
}
else
{
AppointmentModel objAppointmentModel = new AppointmentModel();
if (appointmentModel.AcknowledgementID != Guid.Empty)
{
objAppointmentModel = this._iAppointmentService.GetAppointmentByAckId(appointmentModel.AcknowledgementID);

}
return View(objAppointmentModel);
}

}
Validation method


public List<validationresult> GatePassValidation(Models.AppointmentModel appointmentModel)
{
List<validationresult> gatePassvalidationResultlist = new List<validationresult>();
try
{
if (appointmentModel == null)
{
ValidationResult vr = new ValidationResult();
vr.MemberName = "GlobleError";
vr.Message = "Appointment Record not found";
gatePassvalidationResultlist.Add(vr);
}

if (string.IsNullOrEmpty(appointmentModel.AcknowledgementID.ToString() ))
{
ValidationResult vr = new ValidationResult();
vr.MemberName = "AcknowledgementID";
vr.Message = "Please enter ID";
gatePassvalidationResultlist.Add(vr);
}
}
catch (Exception ex)
{
ValidationResult vr = new ValidationResult();
vr.MemberName = "GlobleError";
vr.Message = "Error : " + ex.Message;
gatePassvalidationResultlist.Add(vr);

}
return gatePassvalidationResultlist;
}


.cshtml

@Html.LabelFor(model => model.VisitorName, new { htmlAttributes = new { @class = "form-control input-sm " } })I am getting error in this line argument null exception was unhandled by user code”