If a designation of a standby guardian is effective at the time of death of the principal, such designation shall remain in effect for a period of ninety days after such death. At the end of such ninety-day period, the authority of such standby guardian shall cease, unless such standby guardian files an application for guardianship with the probate court in the district in which the minor resides and temporary custody of the minor is granted to such standby guardian or the court appoints such standby guardian as guardian of the person of the minor.