private void btnAdd_Click(object sender, RoutedEventArgs e)
        {
            InvoiceViewModel invoice = new InvoiceViewModel();

            invoice.Identifier  = Guid.NewGuid();
            invoice.InvoiceDate = DateTime.Now;

            InvoiceAddEdit addEditForm = new InvoiceAddEdit(invoice, true, false);

            addEditForm.InvoiceCreatedUpdated += new InvoiceHandler(SyncData);
            FlyoutHelper.OpenFlyout(this, ((string)Application.Current.FindResource("Podaci_o_fakturama")), 95, addEditForm);
        }
        private void btnEdit_Click(object sender, RoutedEventArgs e)
        {
            if (CurrentInvoice == null)
            {
                MainWindow.WarningMessage = ((string)Application.Current.FindResource("Morate_odabrati_fakturu_za_izmenuUzvičnik"));
                return;
            }

            InvoiceAddEdit addEditForm = new InvoiceAddEdit(CurrentInvoice, false);

            addEditForm.InvoiceCreatedUpdated += new InvoiceHandler(SyncData);
            FlyoutHelper.OpenFlyout(this, ((string)Application.Current.FindResource("Podaci_o_fakturama")), 95, addEditForm);
        }