Some Bugfixes

Elwig/Windows/PaymentVariantsWindow.xaml.cs

@@ -259,8 +259,12 @@ namespace Elwig.Windows {
                 return;
             CommitButton.IsEnabled = false;
             Mouse.OverrideCursor = Cursors.AppStarting;
-            var b = new BillingVariant(v.Year, v.AvNr);
-            await b.Commit();
+            try {
+                var b = new BillingVariant(v.Year, v.AvNr);
+                await b.Commit();
+            } catch (Exception exc) {
+                MessageBox.Show(exc.Message, "Fehler", MessageBoxButton.OK, MessageBoxImage.Error);
+            }
             Mouse.OverrideCursor = null;
             RevertButton.IsEnabled = true;
             await App.HintContextChange();