Unsubscribe from closing event or window won't close even after the c…

…onfig dialog was closed.

src/Soloplan.WhatsON.GUI/Config/View/ConfigWindow.xaml.cs

@@ -187,6 +187,7 @@ private void WindowClosing(object sender, System.ComponentModel.CancelEventArgs
         return;
       }
 
+      this.Owner.Closing -= this.OwnerClosing;
       this.configurationViewModel.ApplyToSourceAndSave();
     }