如何修复用于表视图的prepareforsegue

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {

    if segue.identifier == "toChat2" {

        guard let navController = segue.destinationViewController as? UINavigationController,
        let chatController = navController.viewControllers.first as? ChatViewController else { return }

        let indexPaths = self.tableView!.indexPathsForSelectedRows! //crashes on this line
        let indexPath = indexPaths[0] as NSIndexPath

        chatController.senderId = FIRAuth.auth()?.currentUser?.uid
        chatController.senderDisplayName = FIRAuth.auth()?.currentUser?.displayName ?? ""
        chatController.friendId = messagesArray[indexPath.row].userId
        chatController.userName = messagesArray[indexPath.row].name
    }
override func prepareForSegue(segue: UIStoryboa