To reproduce:

Add a RadRibbonForm and set the theme to Office2013Light. Start the application and click the backstage button. Click the close button and you will see an exception.

Workaround:

this.RibbonBar.BackstageControl.BackstageElement.TitleBarElement.CloseButton.MouseDown += TitleBarElement_MouseDown;
void TitleBarElement_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
    this.RibbonBar.BackstageControl.BackstageElement.TitleBarElement.CanManageOwnerForm = false;
    this.Close();
}