I am trying to add an item to my list inside viewcontroller but I get this error:

BmsSupportPortalX.Module.BusinessObjects.Database.LG_124_01_QPRODUCT.altMalzemeler.get returned null.

Here is the method I am trying to use to add items to my list inside businessobject:

   LG_124_01_QPRODUCT task = (LG_124_01_QPRODUCT)View.CurrentObject;
   foreach (altMamüller note in e.PopupWindowViewSelectedObjects)
   {
       task.altMalzemeler.Add(note);
       View.Refresh();
   }}

I added constructor to class but it doesn’t work this way too.

    public partial class LG_124_01_QPRODUCT : XPLiteObject
    {

      List<altMamüller> faltMalzemeler;
      [NonPersistent]
      public List<altMamüller> altMalzemeler
      {
          get { return faltMalzemeler; }
          set { SetPropertyValue<List<altMamüller>>(nameof(altMalzemeler), ref faltMalzemeler, value); }
      }

   public LG_124_01_QPRODUCT()
   {
    altMalzemeler = new List<altMamüller>();
   }

    }

How can I solve this problem?