so I am trying to embed a viewController as a child inside a tabBarController, and the place that viewController inside a container delcaread in that tabBarViewController. I can't seem to access my dataModel file (from the child VC) when I add my viewController to the tabBar as child but if i remove it, then the error goes away. This screems as viewcontroller life cycle error to me. can anyone point out why that is?

here's the viewcontroller where error occurs, it happens inside viewDidLoad() where I try to access the dataModel variable. This variable is instantiated inside the scene delegate like this let dataModel = datamodel(), where datamodel() is my datamodel file that loads the music.

import UIKit
import AVFoundation

class PlayerViewController: UIViewController, AVAudioPlayerDelegate {

    var position: Int = 0
    var dataModel: datamodel!
    var timer: Timer?
   @IBOutlet var holder: UIView!

    var player: AVAudioPlayer?
    var playing = false

    //imageView
    var albumCoverImage: UIImageView!

    // Labels
    var gabayLabel: UILabel!
    var gabyaaLabel: UILabel!

    //buttons
    var playPauseButton: UIButton!
    var forwardButton: UIButton!
    var backwardButton: UIButton!
    var repeatButton: UIButton!

    //sliders
    var volumeSlider: UISlider!
    var seekSlider: UISlider!

    //MARK: - viewcontroller life cycle
    override func viewDidLoad() {
        super.viewDidLoad()
        view.backgroundColor = .systemIndigo
        dataModel.loadAudioFiles()
        styleUI()
        dataModel.player?.delegate = self

        timer = Timer.scheduledTimer(timeInterval: 0.1, target: self, selector: #selector(updateSeekSlider), userInfo: nil, repeats: true)

        //gesture recognizer

    }

    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        //fill in if we needed
    }

    override func viewDidLayoutSubviews() {
        if holder.subviews.count == 0 {
        }
    }

    //
    func styleUI() {

        player = dataModel.player
        guard let player = player else{return}
        player.delegate = self
        let gabayPosition = dataModel.gabayArray[dataModel.position]

        //imageView
        albumCoverImage = UIImageView()
        albumCoverImage.translatesAutoresizingMaskIntoConstraints = false
        albumCoverImage.image = UIImage(named: gabayPosition.imageName)
        albumCoverImage.contentMode = .scaleAspectFit
        holder.addSubview(albumCoverImage)

        //Labels
        gabayLabel = UILabel()
        gabayLabel.text = gabayPosition.gabayName
        gabayLabel.translatesAutoresizingMaskIntoConstraints = false
        gabayLabel.backgroundColor = .systemGreen
        gabayLabel.numberOfLines = 0
        gabayLabel.font = UIFont(name: "helvetica-bold", size: 18)
        gabayLabel.textAlignment = .center
        holder.addSubview(gabayLabel)

        gabyaaLabel = UILabel()
        gabyaaLabel.translatesAutoresizingMaskIntoConstraints = false
        gabyaaLabel.backgroundColor = .systemGreen
        gabyaaLabel.text = gabayPosition.gabyaaName
        gabyaaLabel.numberOfLines = 0
        gabyaaLabel.textAlignment = .center
        gabyaaLabel.font = UIFont(name: "helvetica-bold", size: 15)
        holder.addSubview(gabyaaLabel)

        //volume slider
        volumeSlider = UISlider()
//        volumeSlider.maximumValue = Float(player?.duration ?? 0.0)
        volumeSlider.value = 0.1
        volumeSlider.isUserInteractionEnabled = true
        volumeSlider.isOpaque = true
        volumeSlider.translatesAutoresizingMaskIntoConstraints = false
        volumeSlider.addTarget(self, action: #selector(volumeSliderChanged), for: .valueChanged)
        holder.addSubview(volumeSlider)

        seekSlider = UISlider()
        seekSlider.isOpaque = true
        seekSlider.isUserInteractionEnabled = true
        seekSlider.translatesAutoresizingMaskIntoConstraints = false
        seekSlider.minimumValue = 0
        seekSlider.maximumValue = Float(player.duration)
        seekSlider.addTarget(self, action: #selector(seekSliderChanged), for: .valueChanged)
        holder.addSubview(seekSlider)

        //buttons
        playPauseButton = UIButton()
        playPauseButton.translatesAutoresizingMaskIntoConstraints = false
        playPauseButton.setBackgroundImage(UIImage(systemName: "pause.circle"), for: .normal)
        playPauseButton.addTarget(self, action: #selector(playPauseButtonTapped), for: .touchUpInside)
        holder.addSubview(playPauseButton)

        backwardButton = UIButton()
        backwardButton.translatesAutoresizingMaskIntoConstraints = false
        backwardButton.setBackgroundImage(UIImage(systemName: "backward.circle"), for: .normal)
        backwardButton.addTarget(self, action: #selector(backwardButtonTapped), for: .touchUpInside)

        holder.addSubview(backwardButton)

        forwardButton = UIButton()
        forwardButton.translatesAutoresizingMaskIntoConstraints = false
        forwardButton.setBackgroundImage(UIImage(systemName: "forward.circle"), for: .normal)
        forwardButton.addTarget(self, action: #selector(forwardButtonTapped), for: .touchUpInside)

        holder.addSubview(forwardButton)

        NSLayoutConstraint.activate([
           //cover image
            albumCoverImage.topAnchor.constraint(equalTo: holder.layoutMarginsGuide.topAnchor, constant: 20),
            albumCoverImage.leadingAnchor.constraint(equalTo: holder.leadingAnchor, constant: 0),
            albumCoverImage.trailingAnchor.constraint(equalTo: holder.trailingAnchor, constant: 0),

            //labels
            gabayLabel.topAnchor.constraint(equalTo: albumCoverImage.bottomAnchor, constant: 30),
            gabayLabel.leadingAnchor.constraint(equalTo: holder.layoutMarginsGuide.leadingAnchor, constant: 0),
            gabayLabel.trailingAnchor.constraint(equalTo: holder.layoutMarginsGuide.trailingAnchor, constant: 0),
            gabyaaLabel.centerXAnchor.constraint(equalTo: holder.centerXAnchor),

            gabyaaLabel.topAnchor.constraint(equalTo: gabayLabel.bottomAnchor, constant: 10),
            gabyaaLabel.leadingAnchor.constraint(equalTo: holder.layoutMarginsGuide.leadingAnchor, constant: 0),
            gabyaaLabel.trailingAnchor.constraint(equalTo: holder.layoutMarginsGuide.trailingAnchor, constant: 0),
            gabyaaLabel.centerXAnchor.constraint(equalTo: holder.centerXAnchor),

            //volume slider
            volumeSlider.topAnchor.constraint(equalTo: gabyaaLabel.bottomAnchor, constant: 45),
            volumeSlider.leadingAnchor.constraint(equalTo: holder.layoutMarginsGuide.leadingAnchor, constant: 15),
            volumeSlider.trailingAnchor.constraint(equalTo: holder.layoutMarginsGuide.trailingAnchor, constant: -15),

            //buttons
            playPauseButton.topAnchor.constraint(equalTo: volumeSlider.bottomAnchor, constant: 45),
            playPauseButton.centerXAnchor.constraint(equalTo: holder.centerXAnchor),
            playPauseButton.widthAnchor.constraint(equalToConstant: 50),
            playPauseButton.heightAnchor.constraint(equalToConstant: 50),

            backwardButton.topAnchor.constraint(equalTo: volumeSlider.bottomAnchor, constant: 45),
            backwardButton.leadingAnchor.constraint(equalTo: holder.leadingAnchor, constant: 15),
            backwardButton.widthAnchor.constraint(equalToConstant: 50),
            backwardButton.heightAnchor.constraint(equalToConstant: 50),

            forwardButton.topAnchor.constraint(equalTo: volumeSlider.bottomAnchor, constant: 45),
            forwardButton.trailingAnchor.constraint(equalTo: holder.trailingAnchor, constant: -15),
            forwardButton.widthAnchor.constraint(equalToConstant: 50),
            forwardButton.heightAnchor.constraint(equalToConstant: 50),

            //seek slider
            seekSlider.topAnchor.constraint(equalTo: forwardButton.bottomAnchor, constant: 45),
            seekSlider.leadingAnchor.constraint(equalTo: holder.layoutMarginsGuide.leadingAnchor, constant: 15),
            seekSlider.trailingAnchor.constraint(equalTo: holder.layoutMarginsGuide.trailingAnchor, constant: -15),

        ])

    }

    //button actions

    @objc func playPauseButtonTapped(){
        player = dataModel.player
        guard let player = player else{return}
        if player.isPlaying{
                    player.stop()
                    playPauseButton.setBackgroundImage(UIImage(systemName: "play.circle"), for: .normal)
                }
        else{
            player.play()
            playPauseButton.setBackgroundImage(UIImage(systemName: "pause.circle"), for: .normal)
        }

        print("inside the playpause button")
    }

    @objc func forwardButtonTapped(){
        if dataModel.position < dataModel.gabayArray.count - 1 {
            dataModel.position = dataModel.position + 1
            player?.stop()

            for subview in holder.subviews{
                subview.removeFromSuperview()
            }
            styleUI()
            dataModel.loadAudioFiles()

        }
    }

    @objc func backwardButtonTapped(){
        if dataModel.position > 0 {
            dataModel.position = dataModel.position - 1
            player?.stop()

            for subview in holder.subviews{
                subview.removeFromSuperview()
            }
            dataModel.loadAudioFiles()
            styleUI()
        }
    }

    //sliders actions

    @objc func volumeSliderChanged(_ slider: UISlider){

        player = dataModel.player
        guard let player = player else{return}
        player.prepareToPlay()
        player.volume = slider.value
    }

    @objc func seekSliderChanged(_ slider: UISlider){
        player = dataModel.player

        guard let player = player else{return}
        player.prepareToPlay()
        player.currentTime = TimeInterval(slider.value)
    }

    @objc func updateSeekSlider(){
        player = dataModel.player
        guard let player = player else{return}

        seekSlider.value = Float(player.currentTime)
    }

    //avaudioplayer delegate

    func audioPlayerDidFinishPlaying(_ player: AVAudioPlayer, successfully flag: Bool) {

        if flag, dataModel.position < dataModel.gabayArray.count - 1 {
            dataModel.position += 1

            for subview in holder.subviews {
                subview.removeFromSuperview()
            }

            dataModel.loadAudioFiles()
            styleUI()
        }
        else{
            dataModel.position = 0
            for subview in holder.subviews{
                subview.removeFromSuperview()
            }
            dataModel.loadAudioFiles()
            styleUI()
        }
    }

}

and here's the tabBarViewController where I add the above VC as child and in the container

import UIKit

class TabBarViewController: UITabBarController {

    lazy var miniPlayer: PlayerViewController = {
        let miniPlayerVC = PlayerViewController()
        miniPlayerVC.view.translatesAutoresizingMaskIntoConstraints = false
        return miniPlayerVC
    }()

    lazy var containerView: UIView = {
        let uiView = UIView()
        uiView.translatesAutoresizingMaskIntoConstraints = false
        return uiView
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        addChildViews()
        setConstraints()

    }

    func addChildViews(){
        view.addSubview(containerView)
        addChild(miniPlayer)
        containerView.addSubview(miniPlayer.view)
        didMove(toParent: self)
    }

    func setConstraints(){
        NSLayoutConstraint.activate([
            containerView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            containerView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
            containerView.bottomAnchor.constraint(equalTo: tabBar.topAnchor),
            containerView.heightAnchor.constraint(equalToConstant: 70.0),

            miniPlayer.view.leadingAnchor.constraint(equalTo: containerView.leadingAnchor),
            miniPlayer.view.trailingAnchor.constraint(equalTo: containerView.trailingAnchor),
            miniPlayer.view.topAnchor.constraint(equalTo: containerView.topAnchor),
            miniPlayer.view.bottomAnchor.constraint(equalTo: containerView.bottomAnchor)

        ])
    }

}

Where do you set the dataModel property for the player view controller? I don't see where you set dataModel in the code you showed.

In the code you've shown, you declared a dataModel property as an implicitly unwrapped optional.

var dataModel: datamodel!

But you don't give dataModel an initial value. In your viewDidLoad function, you access dataModel. The value of dataModel is nil so your app crashes with the "Unexpectedly found nil while implicitly unwrapping an Optional value" error.

I recommend setting the dataModel property's type to datamodel instead of datamodel! and set its value before you use it. That will make the error go away.

var dataModel: datamodel

By the way, most Swift programmers capitalize the names of structs and classes, DataModel instead of datamodel.