protected override void OnModelItemChanged(object newItem)
 {
     base.OnModelItemChanged(newItem);
     var mi = newItem as ModelItem;
     if (mi != null)
     {
         argBinder = new ActivityArgumentBinder(mi);
     }
     else
     {
         argBinder = null;
     }
 }
        protected override void OnModelItemChanged(object newItem)
        {
            base.OnModelItemChanged(newItem);

            if (ModelItem != null)
            {
                binder = new ActivityArgumentBinder(ModelItem);
            }
            else
            {
                binder = null;
            }
        }