Application fails to display validation message box for args

[maraf]

A current implementation of Navigator uses Application.Current.MainWindow to show message box, but this property can be null when validating input args.

Exception Info: System.ArgumentNullException
   at System.Windows.Interop.WindowInteropHelper..ctor(System.Windows.Window)
   at System.Windows.MessageBox.Show(System.Windows.Window, System.String, System.String, System.Windows.MessageBoxButton, System.Windows.MessageBoxImage)
   at PackageManager.Services.Navigator.Confirm(System.String, System.String, MessageType)
   at PackageManager.Exceptions.MessageExceptionHandler.Handle(Neptuo.Exceptions.Handlers.IExceptionHandlerContext`1<System.Exception>)
   at Neptuo.Exceptions.Handlers.ExceptionHandlerBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Handle(System.Exception, Neptuo.Collections.Specialized.IKeyValueCollection)
   at Neptuo.Exceptions.Handlers.ExceptionHandlerBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Neptuo.Exceptions.Handlers.IExceptionHandler<Neptuo.Exceptions.Handlers.IExceptionHandlerContext>.Handle(Neptuo.Exceptions.Handlers.IExceptionHandlerContext)
   at Neptuo.Exceptions.Handlers.ExceptionHandlerBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Handle(System.Exception, Neptuo.Collections.Specialized.IKeyValueCollection)
   at Neptuo.Exceptions.Handlers.ExceptionHandlerBuilder`1[[System.__Canon, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]].Neptuo.Exceptions.Handlers.IExceptionHandler.Handle(System.Exception)
   at PackageManager.App.OnDispatcherUnhandledException(System.Object, System.Windows.Threading.DispatcherUnhandledExceptionEventArgs)
   at System.Windows.Threading.Dispatcher.CatchException(System.Exception)