Error clicking Button

Good morning!

I'm studying the Swift language for iOS and I have the following problem. I made a test application, available in the book "Swift - Programe for iPhone and iPad" of the Code House.

It is an application where I put a food and a note for it, and when you click add, it will appear on the screen in a list. The problem is that clicking the "Add" button displays the following error, in the class line, in the AppDelegate.swift file

  

Thread 1: SIGBRT signal

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?

Follow the codes:

ViewController

import UIKit

class ViewController: UIViewController {

@IBOutlet var nameField: UITextField!
@IBOutlet var happinessField: UITextField!

@IBAction func add() {
    if nameField == nil || happinessField == nil {
        return
    }

    let name = nameField!.text
    let happiness = Int(happinessField!.text!)
    if happiness == nil {
        return }
    let meal = Meal(name: name!, happiness: happiness!)
    print("eaten: \(meal.name) \(meal.happiness)")

}
}

Meal:

import Foundation

class Meal {

let name: String
let happiness: Int
var items = Array<Item>()
init(name: String, happiness: Int) {
    self.name = name
    self.happiness = happiness
}

func allCalories() -> Double {
    print("Calculating")
    var total = 0.0
    for i in items {
        total += i.calories
    }
    return total
}

}

Item:

import Foundation

class Item {
let name: String
let calories: Double
init(name: String, calories: Double) {
    self.name = name
    self.calories = calories
}
}