Close command needed when using the close button

The command could be intersected as follow: 
 static MainWindow()
{
      RadRibbonWindow.IsWindowsThemeEnabled = false;
       CommandManager.RegisterClassCommandBinding(typeof(MainWindow), new CommandBinding(Telerik.Windows.Controls.RibbonView.Shell.SystemCommands.CloseWindowCommand, CloseWindowExecuted, canExecute));
}

Please note that this command is available only if RadRibbonWindow.IsWindowsThemeEnabled is set to "false".

Available in R1 2017 Release