Populate parent and nested child class inside for loop using C#

I have two class parent and child list class which i want to populate from controller; i have some problem when it says object null when i loop data to fill that child list

I hope someone can help me

Thanks Ruben

public class Milestone
{
    public long shipment_id
    {
        get;
        set;
    }
    public string vehicle_name
    {
        get;
        set;
    }
    public DateTime? shipment_date
    {
        get;
        set;
    }
    public string shipment_number
    {
        get;
        set;
    }
    public string origin_geofence_code
    {
        get;
        set;
    }
    public DateTime? planned_departure_time
    {
        get;
        set;
    }
    public DateTime? departure_time
    {
        get;
        set;
    }
    public byte? progress
    {
        get;
        set;
    }
    public string driver_name
    {
        get;
        set;
    }
    public List <MilestoneDetail> detail { get; set; }
}
public class MilestoneDetail
{
    public long? shipment_destination_id
    {
        get;
        set;
    }
 
    public short? number
    {
        get;
        set;
    }
    public string destination_geofence_name
    {
        get;
        set;
    }
    public DateTime? arrival_time
    {
        get;
        set;
    }
 
    public DateTime? departure_time
    {
        get;
        set;
    }
 
    public DateTime? planned_arrival_time
    {
        get;
        set;
    }
 
    public DateTime? planned_departure_time
    {
        get;
        set;
    }
}

[HttpPost]
public JsonResult GetMilestone(DateTime fromDate, DateTime toDate, string search)
{
    List<Milestone> model = new List<Milestone>();
    List<ViewShipmentSummary> dt = db.ViewShipmentSummaries.Where(m => m.shipment_date >= fromDate && m.shipment_date <= toDate).ToList();
    foreach (var i in dt)
    {
        Milestone ml = new Milestone();
        ml.shipment_id = i.shipment_id;
        ml.vehicle_name = i.vehicle_name;
        ml.shipment_number = i.shipment_number;
        ml.shipment_date = i.shipment_date;
        ml.origin_geofence_code = i.origin_geofence_name;
        ml.planned_departure_time = i.planned_departure_time;
        ml.departure_time = i.departure_time;
        ml.driver_name = i.driver_name;
        ml.progress = i.progress;
        model.Add(ml);
        var des = db.ViewShipmentDestinations.Where(m => m.shipment_id == i.shipment_id).OrderBy(m => m.number).ToList();
        foreach(var a in des)
        {
            MilestoneDetail mdetail = new MilestoneDetail();
            mdetail.shipment_destination_id = a.shipment_destination_id;
            mdetail.number = a.number;
            mdetail.destination_geofence_name = a.destination_geofence_name;
            mdetail.planned_departure_time = a.planned_departure_time;
            mdetail.departure_time = a.departure_time;
            mdetail.planned_arrival_time = a.planned_arrival_time;
            mdetail.arrival_time = a.arrival_time;
            ml.detail.Add(mdetail);
        }
    }     
    return Json(model, JsonRequestBehavior.AllowGet);
}

The error that i get is "System.NullReferenceException: 'Object reference not set to an instance of an object.'"

I got error at line 30 when try to add mdetail to ml.detail