MessageKitでチャットが1度は表示されるのですが再びロードするとクラッシュします

前提・実現したいこと

MessageKitを用いてチャット機能を実装しようとしています。現在チャット一覧からチャットルームをクリックするとMessagesCollectionViewにメッセージが表示出来るのですが、その後チャットルームを退出し、チャットルーム一覧に戻って同じチャットルームをクリックすると以下のエラーメッセージが表示されクラッシュします。

発生している問題・エラーメッセージ

Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value: file

というエラーが

func currentSender() -> SenderType {
        return currentUser
}

のreturn currentUserに対して表示され原因が分かりません。
###ソースコード

import UIKit
import MessageKit
import MessageInputBar
import InputBarAccessoryView
import FirebaseFirestore
import FirebaseAuth
import FirebaseStorage
import Nuke

struct Sender: SenderType{
    var senderId: String
    var displayName: String
    
    
    init(dic: [String: Any]){
        
        self.senderId = dic["senderId"] as? String ?? ""
        self.displayName = dic["displayName"] as? String ?? ""
        
    }
   
    
}


struct MessageKitMessage: MessageType{
    var sender: SenderType
    var messageId: String
    let sentDate: Date
    var kind: MessageKind
    
//    init(dic: [String: Any]){
//
//        self.sender = Sender(dic: dic)
//        self.messageId = dic["messageId"] as? String ?? ""
//        self.kind = dic["kind"] as! MessageKind
//        self.sentDate = dic["senderId"] as! Date
//
//
//    }
    
}


class MessageKitChatViewController: MessagesViewController, InputBarAccessoryViewDelegate, MessageInputBarDelegate, MessagesDataSource{
    
    
    
    var senderUser = Auth.auth().currentUser!
    var messages = [MessageType]()
    
    
//    let currentUser = Sender(senderId: Auth.auth().currentUser!.uid, displayName: Auth.auth().currentUser!.displayName ??  "Name not found")
    var passedId: String!
    var passedDisplayName: String!
    
    var currentUser: Sender! = nil
    var otherUser: Sender! = nil
   
    
    var passedChatroomId: String!
    
    
   

    override func viewDidLoad() {
        super.viewDidLoad()
        
        messagesCollectionView.messagesDataSource = self
        messagesCollectionView.messagesLayoutDelegate = self
        messagesCollectionView.messagesDisplayDelegate = self
        messageInputBar.delegate = self
         
        
//        otherUser = Sender(senderId: passedId, displayName: passedDisplayName)
        
        
    }
    
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        
        
        loadBothUsersInfo {
            self.loadMessages()
        }
        
    }
    func loadBothUsersInfo(completion: @escaping () -> Void) {
        guard let currentUserId = Auth.auth().currentUser?.uid else { return }
        print("currentUserId")
        print(currentUserId)
        print("currentUserId")
        Firestore.firestore().collection("users").document(currentUserId).getDocument { (snapshot, error) in
            if error != nil{
                print(error)
            }else{
                guard let dic = snapshot?.data() else { return }
                let user = User(dic: dic)
                let currentUserName = user.userName
               
//                self.currentUser = Sender(senderId: currentUserId, displayName: currentUserName)
                
                let dic2 = ["senderId": dic["userId"], "displayName": dic["userName"]]
                
                self.currentUser = Sender(dic: dic2 as [String : Any])
                
                
            }
        }
        
        Firestore.firestore().collection("users").document(passedId).getDocument { (snapshot, error) in
            if error != nil{
                print(error)
            }else{
                guard let dic = snapshot?.data() else { return }
                let user = User(dic: dic)
                let otherUserName = user.userName
                let dic2 = ["senderId": dic["userId"], "displayName": dic["userName"]]
                self.otherUser = Sender(dic: dic2)
                
            }
        }
        completion()
    }
    
    
    func inputBar(_ inputBar: InputBarAccessoryView, didPressSendButtonWith text: String) {
        

        Firestore.firestore().collection("chatRooms").document(passedChatroomId).collection("messages").addDocument(data: ["senderId": currentUser.senderId, "displayName": currentUser.displayName, "messageId": "", "sentDate": Timestamp().dateValue(), "kind": text]) { (error) in
            if error != nil{
                print(error)
            }else{
                print("メッセージの保存、送信が完了")
                inputBar.inputTextView.text = nil
                self.messagesCollectionView.reloadData()
                
                self.messagesCollectionView.scrollToLastItem()
                
            }
        }
        
       
    }
    
    func loadMessages(){
        Firestore.firestore().collection("chatRooms").document(passedChatroomId).collection("messages").getDocuments { (snapshots, error) in
            if error != nil{
                print(error)
            }else{
                snapshots?.documents.forEach({ (snapshot) in
                    
                    
                    let dic = snapshot.data()
                   
                    let kind = dic["kind"]!
                    let senderId = dic["senderId"]!
                    let sentDate = dic["sentDate"]! as! Timestamp
                    let convertedSentDate = sentDate.dateValue()
                    let displayName = dic["displayName"]!
                    let messageId = dic["messageId"]!
                    
                    let sender = Sender(dic: dic)
                    
                    let dic2 = ["sender": sender, "messageId": dic["messageId"]!, "sentDate": convertedSentDate, "kind": MessageKind.text(dic["kind"] as! String)]
//                    print("'''''''''''''''")
//                    print(sender)
//                    print("'''''''''''''''")
//                    print(kind)
//                    print(senderId)
                    print("dic2")
                    print(dic2)
                    print("dic2")
//                    print(displayName)
//                    print(messageId)
                    self.messages.append(MessageKitMessage(sender: sender, messageId: messageId as! String, sentDate: convertedSentDate, kind: MessageKind.text(dic["kind"] as! String)))
                    self.messagesCollectionView.reloadData()
//                    self.messagesCollectionView.scrollToLastItem()
                    
//                    MessageKitMessage(sender: sender, messageId: dic["messageId"] as! String, sentDate: dic["sentDate"], kind: dic["kind"])
                    
//                    self.messages.append(MessageKitMessage(dic: dic))
                    print("messages")
                    print(self.messages)
                    print("messages")
//                    MessageKitMessage(sender: sender, messageId: dic["messageId"] as! String, sentDate: sentDate as! Date, kind: kind)

//                    let sender = Sender(senderId: senderId!, displayName: displayName!)
//                    if senderId != nil {
//                        if displayName != nil{
//                            let sender = Sender(senderId: senderId!, displayName: displayName!)
//                        }
//
//                    }
                    
//                        Sender(senderId: senderId, displayName: displayName)
                    
//                    self.messages.append(MessageKitMessage(dic: dic))
                    
                   
                    
                    
                })
            }
        }
    }
    
    
    func currentSender() -> SenderType {
        return currentUser
    }
    
    func messageForItem(at indexPath: IndexPath, in messagesCollectionView: MessagesCollectionView) -> MessageType {
        
        return messages[indexPath.section]
    }
    
    func numberOfSections(in messagesCollectionView: MessagesCollectionView) -> Int {
        return messages.count
    }

    
}


//
//extension MessageKitChatViewController: MessagesDataSource{
//
//
//    func currentSender() -> SenderType {
//            return currentUser
//        }
//
//        func messageForItem(at indexPath: IndexPath, in messagesCollectionView: MessagesCollectionView) -> MessageType {
//            print("000000000000000")
//            print(messages[indexPath.section].sentDate)
//            print(messages.count)
//            print("000000000000000")
//            return messages[indexPath.section]
//        }
//
//        func numberOfSections(in messagesCollectionView: MessagesCollectionView) -> Int {
//            return messages.count
//        }
//
//
//
//
//}


extension MessageKitChatViewController: MessagesLayoutDelegate{
    
    
}

extension MessageKitChatViewController: MessagesDisplayDelegate{
    
    
}