Delegate está nulo

Olá, meu código parou de funcionar. Usando alguns Prints, vi que o delegate está nulo e o app parou de funcionar, ou seja, ao clicar em "enviar", o app nao volta para a tela anterior.

//
//  ViewController.swift
//  Primeiro-Projeto-Alura
//
//  Created by Douglas B de souza on 16/07/17.
//  Copyright © 2017 Douglas B de souza. All rights reserved.
//

import UIKit

class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {

    @IBOutlet var nameField : UITextField?
    @IBOutlet var happinessField : UITextField?
    var delegate : AddAMealDelegate?
    var selected = Array<Item>()

    var items = [
        Item(name: "Eggplant", calories: 10),
        Item(name: "Brownie", calories: 10),
        Item(name: "Zucchini", calories: 10),
        Item(name: "Muffin", calories: 10),
        Item(name: "Coconut oil", calories: 500),
        Item(name: "Chocolate frosting", calories: 1000),
        Item(name: "Chocolate chip", calories: 1000)
    ]

    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {

        if let cell = tableView.cellForRow(at: indexPath) {
            if (cell.accessoryType == UITableViewCellAccessoryType.none) {
                cell.accessoryType = UITableViewCellAccessoryType.checkmark
                let item = items[indexPath.row]
                selected.append(item)
            }
            else {
                cell.accessoryType = UITableViewCellAccessoryType.none
                let item = items[indexPath.row]
                if let position = selected.index(of: item) {
                    selected.remove(at: position)
                }
            }
        }
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return items.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: UITableViewCellStyle.default, reuseIdentifier: nil)
        let row = indexPath.row
        let item = items[row]
        cell.textLabel!.text = item.name
        return cell
    }



    @IBAction func add(){

        if(nameField == nil || happinessField == nil){
            return
        }

        let name:String = nameField!.text!

        if let happiness = Int(happinessField!.text!){

            let meal = Meal(name: name, happiness: happiness, items: selected)

            print("eaten \(meal.name) with happiness \(String(meal.happiness))! with \(meal.items)!")

            if (delegate == nil) {
                return
            }

            delegate!.add(meal)

            if let navigation = navigationController {
                navigation.popViewController(animated: true)
            }
        }
    }
}

Aqui, segue o meu AddAMealDelegate

import Foundation

protocol AddAMealDelegate {
    func add(_ meal: Meal)
}