If a constitutional officer who has appointed successors as provided in this chapter becomes unavailable because of a disaster, the powers and duties of the officer’s office shall devolve upon one of the officer’s appointees in the order specified in making the appointments and that person shall declare that that person is undertaking the duties of the office and take and subscribe the oath therefor; provided, however, that an appointee so designated may declare that the appointee 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.