MainWindow.xaml.cs

private void CloseApp()
{
    Environment.Exit(0); //поработать над кодами ошибок!
}

private void ExitButton_Click(object sender, RoutedEventArgs e)
{
    CloseApp();
}

private void ShowWindow(object sender, RoutedEventArgs e)
{
    this.Show();
    //this.Activate();
    this.Focus();
    this.BringIntoView();
}