Death, etc. of Governor before oath of office
Section 181(2) · 1999 Constitution (2004 edition)
What Constitution says
(2) Where the persons duly elected as Governor and Deputy Governor of a State die or are for any reason unable to assume office before the inauguration of the house of Assembly, the Independent National Electoral Commission shall immediately conduct an election for a Governor and Deputy Governor of the State.
This is the 1999 Constitution as published in 2004. Later amendments aren't included yet, so a section may have changed since.
Original source
Official Gazette · CFRN 1999 official PDF · printed page 85. Original document
Release: draft-c8d50d87fae9. Source scope, review and corrections