salesItem = (from SalesItem items in this.ProductList.ItemsSource
                             select items).ToList<Sales>();

this.ProductList.ItemsSource = Enumerable.Empty<Sales>();

if (this.ProductList != null && this.ProductList.ItemsSource != null)
{
    salesItem = (from SalesItem items in this.ProductList.ItemsSource
                             select items).ToList<Sales>();
}