A claim for workers’ compensation benefits shall be deemed to be in the course of employment with the Virginia National Guard or Virginia Defense Force for any member thereof who, reacting to an order to report received while he is outside an assigned shift or work location, undertakes in direct obedience to a lawful military order travel by the most expeditious route to his designated place of state active duty pursuant to §§ 44-54.4, 44-75.1, and 44-78.1. Nothing in this section shall prohibit an employer from using any defense otherwise available under this title.