unread text message

I got a problem, I got an unread text message and I'm not even able to read it, I click on it and a chat window opens with nothing in there the nickname is (unassigned) and ♥♥♥♥ like that how do I remove the unread chat message I've had that ♥♥♥♥ for a long time now it is really annoying

unread text message