Swift Thread 1: signal SIGART error after adding a new delegate - swift

Question

I’m new to Swift and have been stuck on this problem for a while.
I’m trying to add a ‘settings’ view where the size of the font can be updated for the main view.
I get a Thread 1: signal SIGART error on the line: class AppDelegate: UIResponder, UIApplicationDelegate
After adding the below code, my app now crashes when I press on the settings button.
Not sure how else to debug this problem, any help will be much appreciated! Thanks in advance.
View Controller:
class ViewController: UIViewController, CLLocationManagerDelegate, SettingsViewControllerDelegate {
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "SettingsViewController" {
let settingsViewController = segue.destination as? SettingsViewController
if let viewController = settingsViewController {
viewController.delegate = self
}
}
}
func makeChangesToFont(size: Int) {
if (self.distanceReading.font.pointSize > 20) {
self.distanceReading.font = UIFont(name: self.distanceReading.font.fontName, size: CGFloat(size))
}
}
...
}
Settings Controller:
import UIKit
protocol SettingsViewControllerDelegate {
func makeChangesToFont(size: Int)
}
class SettingsViewController: UIViewController {
var delegate: SettingsViewControllerDelegate?
#IBAction func IncreaseSize(_ sender: UIButton) {
print("something")
increaseFontSize()
}
func increaseFontSize() {
let mainView = parent as! ViewController
let text = mainView.distanceReading
let fontSize = (text?.font.pointSize)! + 10
print(fontSize)
if let delegate = self.delegate {
delegate.makeChangesToFont(size: Int(fontSize))
}
}
...
}
I'm using Swift 3 in Xcode 8.2.1