The MINAStatCollector is an Openfire class that is designed to integrate with Mina and collect runtime statistics for C2S sessions. Based on the code, I think the only way you could get this exception is if somehow the removeSession() method was called before the addSession() method finished executing. Given that the Mina library is highly asynchronous, I suppose this is possible under certain unusual conditions (high load, network latency, etc.).
Opened OF-878 to track.