private void InvoiceDG_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
if (InvoiceDG.SelectedItem != null)
{
FC_Invoice SelectedInvoice = (FC_Invoice)InvoiceDG.SelectedItem;
ContractsPreInvouce.ItemsSource = PlannerClass.GetContracts_PreInvoice(SelectedInvoice);
}
}