App crashing when launching

let vendingMachine: VendingMachineType

required init?(coder aDecoder: NSCoder) {
    do {
        let dictionary = try PlistConverter.dictionaryFromFile("VendingInventory", ofType: "plist")
        let inventory = try InventoryUnarchiver.vendingInventoryFromDictionary(dictionary)
        self.vendingMachine = VendingMachine(inventory: inventory)
    } catch let error {
        fatalError("\(error)")
    }
    super.init(coder: aDecoder)
}

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    setupCollectionViewCells()
    print(vendingMachine.inventory)
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

// MARK: - UICollectionView 

func setupCollectionViewCells() {
    let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
    layout.sectionInset = UIEdgeInsets(top: 20, left: 0, bottom: 10, right: 0)
    let padding: CGFloat = 10
    layout.itemSize = CGSize(width: (screenWidth / 3) - padding, height: (screenWidth / 3) - padding)
    layout.minimumInteritemSpacing = 10
    layout.minimumLineSpacing = 10

    collectionView.collectionViewLayout = layout
}

func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 12
}