Just to be clear... I just checked this in my FC. I can promote people up to the rank under FC master. I cannot demote the master, nor can I promote someone to master. So this DOES NOT fix an instance where the master goes missing but does not actually leave the guild. The only instance I can think of it fixing is if someone physically leaves an FC without first appointing someone. I can't test if leaving is actually possible without first doing that because I'm not the Master.

Why not just make the FC leader promote someone new before leaving?