Went into trouble right at the start of Apple Education Develop with Swift Fundamentals

Hello fellows I am studying iOS development with Apple’s Education book Develop with Swift Fundamentals Xcode12 and went into peculiar crush right into the beginning of the course about IBOutlets and IBActions.

So during the lab I was needed to create UILabel and UIButton that changes UILabel on the press but the program crushes during runtime with “Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value”. I do get its crashes due to nil optional unwrapping but I don’t get why since I a bit newbie here :(

My ViewController code is below

 //
//  ViewController.swift
//  InterfaceBuilderBasics
//
//  Created by Shun Kaido on 09.11.2021.
//

import UIKit

class ViewController: UIViewController {

    @IBOutlet var mainLabel: UILabel!
        
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }

    @IBAction func changeTitle(_ sender: Any) {
        mainLabel.text = "This app rocks!"
    }
   

}