signal SIGABRT ('NSUnknownKeyException') error when trying to run Xcode 10.3 app [Not using Storyboards]

Question

.xib file (Interface), Show the Identity Inspector Tab I am new to Xcode and Swift, and was trying to run a simple multiplication game app. However, when I try to run a test on the simulator, I receive a SIGABRT signal error, and the app does not load. I am not sure how to approach this, please help! I have tries everything and nothing seems to work. Note: I am not using storyboards, and have deleted the main.storyboard file from the project folder, focusing on the xib file (I am using Xcode 10.3)

The Error I receive is:

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key inputField.'

// This is my AppDelegate. I receive the SIGABRT signal on the third line (class AppDelegate). 


import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?
    var mainVC: MainViewController?

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {

        mainVC = MainViewController(nibName: "MainViewController", bundle:nil)

        let frame = UIScreen.main.bounds
        window = UIWindow(frame:frame)

        window!.rootViewController = mainVC
        window!.makeKeyAndVisible()

        return true
    }

}

---

Separate file

//MainViewController.swift, created with MainViewController.xib

import UIKit

class MainViewController: UIViewController
{   
@IBOutlet weak var number1:UILabel?
@IBOutlet weak var number2:UILabel?
@IBOutlet weak var timeLabel:UILabel?
@IBOutlet weak var scorecounter:UILabel?
@IBOutlet weak var inputField:UITextField?

var score:Int = 0
var timer:Timer?
var seconds:Int = 60

override func viewDidLoad()
{
    super.viewDidLoad()
    setRandomNumberLabel1()
    setRandomNumberLabel2()
    updateScoreLabel()

    inputField?.addTarget(self, action: #selector(textFieldDidChange(textField:)), for:UIControl.Event.editingChanged)
}

func updateScoreLabel()
{
    scorecounter?.text = "\(score)"
}

func updateTimeLabel()
{
    if(timeLabel != nil)
    {
        let min:Int = (seconds / 60) % 60
        let sec:Int = seconds % 60

        let min_p:String = String(format: "%02d", min)
        let sec_p:String = String(format: "%02d", sec)

        timeLabel!.text = "\(min_p):\(sec_p)"
    }
}

func setRandomNumberLabel1()
{
    number1?.text = generateRandomString()
}

func setRandomNumberLabel2()
{
    number2?.text = generateRandomString()
}

@objc func textFieldDidChange(textField:UITextField)
{
    if inputField?.text?.count ?? 0 < 1
    {
        return
    }

    if  let number1_text    = number1?.text,
        let number2_text    = number2?.text,
        let input_text      = inputField?.text,
        let number1 = Int(number1_text),
        let number2 = Int(number2_text),
        let input   = Int(input_text)
    {
        print("Comparing: \(input_text) == \(number1_text) times \(number2_text)")

        if(input == number1 * number2)
        {
            print("Correct!")

            score += 1
        }
        else
        {
            print("Incorrect!")

            score -= 1
        }
    }

    setRandomNumberLabel1()
    setRandomNumberLabel2()
    updateScoreLabel()
    updateTimeLabel()

    if(timer == nil)
    {
        timer = Timer.scheduledTimer(timeInterval: 1.0, target:self, selector:#selector(onUpdateTimer), userInfo:nil, repeats:true)
    }
}

@objc func onUpdateTimer() -> Void
{
    if(seconds > 0 && seconds <= 60)
    {
        seconds -= 1

        updateTimeLabel()
    }
    else if(seconds == 0)
    {
        if(timer != nil)
        {
            timer!.invalidate()
            timer = nil

            let alertController = UIAlertController(title: "Game Over!", message: "Time's up! You got a score of: \(score) points. Good Job!", preferredStyle: .alert)

            let restartAction = UIAlertAction(title: "Restart", style: .default, handler: nil)
            alertController.addAction(restartAction)

            self.present(alertController, animated: true, completion: nil)

            score = 0
            seconds = 60

            updateTimeLabel()
            updateScoreLabel()
            setRandomNumberLabel1()
            setRandomNumberLabel2()
        }
    }
}

override func didReceiveMemoryWarning()
{
    super.didReceiveMemoryWarning()
}

func generateRandomString() -> String
{
    var result:String = ""

    for _ in 1...1
    {
        let digit = Int.random(in: 1..<10)

        result += "\(digit)"
    }

    return result
}
}

Answer 1

Change your outlets to be non-optional:

@IBOutlet weak var number1: UILabel!
@IBOutlet weak var number2: UILabel!
@IBOutlet weak var timeLabel: UILabel!
@IBOutlet weak var scorecounter: UILabel!
@IBOutlet weak var inputField: UITextField!

1. Open your viewController in your Storyboard and select your controller:

2. Check the menu on the right. It should have changed to have the last icon, an arrow, selected. Your outlets should be there:

3. Connect your outlets to each element on the view. You can do this by clicking on the empty radio button on each element and dragging the cursor:

4. Build and run the project again.

If by any chance you can't see the outlets in step 2, you might have a wrong reference to your class. After step 1, click on the middle button on the right menu:

If the class field doesn't match your controller, change to it. Based in your code, yours should be MainViewController. Set that and go back to step 2.