BillingVariant: Assume price to be 0 for undefined vaributes

Elwig/Windows/ChartWindow.xaml.cs

@@ -664,8 +664,10 @@ namespace Elwig.Windows {
             try {
                 await Task.Run(async () => {
                     var b = new BillingVariant(PaymentVar.Year, PaymentVar.AvNr);
-                    await b.Calculate();
+                    await b.Calculate(false);
                 });
+            } catch (KeyNotFoundException exc) {
+                MessageBox.Show(exc.Message, "Noch nicht alle Preise festgelegt", MessageBoxButton.OK, MessageBoxImage.Information);
             } catch (Exception exc) {
                 MessageBox.Show(exc.Message, "Berechnungsfehler", MessageBoxButton.OK, MessageBoxImage.Error);
             }