Found nil while unwrapping an Optional value even though there is a value

Question

I'm passing data from one controller to another using a segue. What's strange is that the second view controller is able to detect the data value in one part of the code and throwing a "Unexpectedly found nil while unwrapping an Optional value" error in another part. Can someone help?

View Controller 1: Passing data to View Controller 2

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    if segue.identifier == "showMsg" {
        
       let nav = segue.destination as! UINavigationController
       let VC = nav.topViewController as! MessagesViewController
                if let ref = ref {
                        DatabaseConnect.fetchTransactionUser(ref: ref, completion: { (user) in
                            if let user = user {
                                VC.userId = "Data passed"

                            }
                        })
                }
    }
}

View Controller 2: Retrieving data from View Controller 1

  var userId:String?
  func fetchFeed() {
  
  let message = Database.database().reference().child("allmessages").child(currentUser).child(userId!) //ERROR HERE!
        message.observe(.value, with: { (snapshot) in
                let value = snapshot.value as? NSDictionary
                let postId = value?["postId"] as? String ?? ""
        })
  }

  //meanwhile, later in the file, this action is working below for some reason
    @IBAction func postCommentAction(_ sender: Any) {
    var commentText = ""
    
    if let text = textFieldLabel.text {
        
       commentText = text
        
    }
    
    if commentText != "" {
        
        let currentUser = Auth.auth().currentUser!.uid
                    
        let messageIdExist = Database.database().reference().child("allmessages").child(currentUser).child(userId!) //THIS WORKS!
                    
                    messageIdExist.observeSingleEvent(of: .value, with: { (snapshot) in
                         
                           })