Error on ticket closure issue

Hello all,

I have an issue with ticket status change. While creating tickets, replying, changing statuses to various ones, everything seems to be in order. The issue starts when closing a ticket. Whenever a case is changed to "Closed" attached error pops out.

Does anyone have any idea how to get rid of that annoying popup? The ticket itself is not affected; it just messes up my OCD.

Thank you in advance!