I previously had this issue. It only affects admins who have the rights to change the order of the pinned topics which is why it's unlikely others can't reproduce on the OPs site.
However since 1.7 (could be Chrome 64) I no longer have the problem.
Yeah looks like mainPid is somehow missing you can check the admin>advanced>events log to see if someone deleted post 36210. Can also check if its still in the database by
