iOS 退出聊天室的时候偶尔崩溃

自己开的异步线程，调用的同步方法

    NSString *roomID = [room_id copy];

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

        EMError *error = nil;

        [[EMClient sharedClient].roomManager leaveChatroom:roomID error:&error];

        dispatch_async(dispatch_get_main_queue(), ^{

        });

    });