Error in calling method from another class when using delegates - swift

Question

I have tried to run Second ViewController to change appearance on my first ViewController and progressBar(Progress View) cause error, but if I try to call method keyButtonResponse() from First View Controller error doesn`t appear
First ViewController:
import UIKit
import AVFoundation
import UserNotifications
protocol SomeDelegate {
func keyButtonResponse()
}
class FirstViewController: UIViewController, SomeDelegate{
#IBOutlet weak var label: UILabel!
#IBOutlet weak var progressBar: UIProgressView!
func keyButtonResponse(){
//some code
progressBar.progress = 0.01 //Thread 1: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value
}
}
Second ViewController:
class SecondViewController: UIViewController {
var delegate: SomeDelegate?
func keyButtonResponse(){
delegate?.keyButtonResponse()
}
#IBAction func leftButtonPressed(_ sender: UIButton) {
let myVar = WarningViewController()
let viewController = ViewController()
myVar.delegate = viewController
myVar.keyButtonResponse()
}
}

Answer 1

You're setting the delegate to an instance of a view controller that's unrelated to FirstViewController here.
let viewController = ViewController()
myVar.delegate = viewController
You need to set the delegate to the existing instance of FirstViewController.
If you're using a segue:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if let vc = segue.destination as? SecondViewController, segue.identifier == "firstVCToSecondVCSegue" {
vc.delegate = self
}
}
or
if let vc = self.storyboard?.instantiateViewController(withIdentifier: "SecondViewController") as? SecondViewController {
vc.delegate = self
self.navigationController?.pushViewController(vc, animated: true)
}
This should be placed within FirstViewController, not SecondViewController.