Swift：fetchedResultsController.getter：致命错误：在展开Optional值时意外发现nil

I was doing a chat app, but when running it, it showed that in fetchedResultsController.getter(closure #1),there is a fatal error:unexpectedly found nil while unwrapping an Optional value. I double checked it, but found nothing wrong. Could you help me fix it?I was doing a chat app, but when running it, it