erro na função showNewItem()

estou tendo dificuldade com o conteúdo dessa aula. Tive que usar o "@objc" antes da função "showNewItem()", por um erro de compatibilidade sugerido pela IDE. Na ultima aula, enquanto essa função apenas pintava um algo na tela o código funcionava normalmente, porem depois que eu inseri na função o conteúdo proposto na aula:

let newItem = NewItemViewController()
        if let navigation = navigationController {
            navigation.pushViewController(newItem, animated: true)
        } 

A aplicação começou a crasear sempre que eu clico no botão "add new item" .

Segue todo o código do arquivo ViewController.swift:

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: "cheese", calories: 10),
        Item(name: "pasta", calories: 10),
        Item(name: "bread", calories: 10),
        Item(name: "meat", calories: 10),
        Item(name: "pepperoni", calories: 10)
    ]

    override func viewDidLoad() {
        let newItemButton = UIBarButtonItem(title: "add new item", style: UIBarButtonItem.Style.plain, target: self, action: #selector(showNewItem))
        navigationItem.rightBarButtonItem = newItemButton
    }

    @objc func showNewItem() {
        let newItem = NewItemViewController()
        if let navigation = navigationController {
            navigation.pushViewController(newItem, animated: true)
        }
    }

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

    func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        if let cell = tableView.cellForRow(at: indexPath) {
            if cell.accessoryType == UITableViewCell.AccessoryType.none {
                cell.accessoryType = UITableViewCell.AccessoryType.checkmark
                let item = items[indexPath.row]
                selected.append(item)
            }
            else {
                cell.accessoryType = UITableViewCell.AccessoryType.none
                let item = items[indexPath.row]
                if let position = selected.index(of: item) {
                    selected.remove(at: position)
                }
            }
        }
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let row = indexPath.row
        let item = items[row]
        let cell = UITableViewCell(style: UITableViewCell.CellStyle.default, reuseIdentifier: nil)
        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 \(meal.happiness) with \(selected)")

            if (delegate == nil) {
                return
            }

            delegate!.add(meal)

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