If any constitutional officer who has appointed successors as provided in this chapter becomes unavailable because of a disaster, the powers and duties of his office shall devolve upon one of his appointees in the order which he specified in making the appointments and such person shall declare that he is undertaking the duties of the office and take and subscribe the oath therefor; provided, however, that any appointee so designated may declare that he is undertaking the office and take the prescribed oath if no person prior in such order of succession enters upon the office within seven days after the incumbent thereof becomes unavailable.