Fix assertion in notification clearing; fix #6574
Fix assertion in notification clearing; fix #6574

Hopefully, this fixes ALL the issues with this code.

kpiwko/geary,aatishnn/geary,martinra/geary,geekvigilante/geary,emersion/geary,rschroll/geary,glebihan/geary,emersion/geary,githubnemo/geary,AtticHacker/geary,kyoushuu/geary,aatishnn/geary,tshikaboom/geary,githubnemo/geary,glebihan/geary,tshikaboom/geary,mjog/geary,robol/geary,MohamedIM/geary-unofficial,aatishnn/geary,mjog/geary,mjog/geary,martinra/geary,mpdeimos/geary,AtticHacker/geary,kyoushuu/geary,githubnemo/geary,tshikaboom/geary,jimbonator/geary,AtticHacker/geary,jimbonator/geary,robol/geary,abn/geary,jimbonator/geary,emersion/geary,robol/geary,rschroll/geary,glebihan/geary,martinra/geary