How to show a random image from an array into another swift file (this case a view controller)?

I'm trying to show a random image into a view controller, although the function of creating the random image is in another swift-file (and it should be like that!). Now I'm getting the error: "Unexpectedly found nil while unwrapping an Optional value" when I'm assigning the random image to the image view of the viewcontroller (line 3 of the function 'createRandomOutfit()'. I honestly don't know why and how I should solve this. I hope someone can help me!

p.s. I also added the images into the same folder as the view controller.

//
//  Wardrobe.swift
//  WhatToWear
//
//  Created by issd on 06/04/2018.
//  Copyright © 2018 1. All rights reserved.
//

import Foundation

var allCoats = ["coat3.jpg", "coat6.jpg", "coat12.jpg", "coat21.jpg"]
var allSweaters = ["sweater1.jpg", "sweater5.jpg", "sweater10.jpg", "sweater21.jpg"]
var allTrousers = ["trousers7.jpg", "trousers11.jpg", "trousers14.jpeg", "trousers24.jpeg"]
var allShoes = ["shoe5.png", "shoe10.jpg", "shoe16.jpg", "shoe25.jpg"]

var outfitViewController = ViewController()

func createRandomOutfit() -> String  {

    var outfitResult = Outfit(coat: nil, sweater: nil, trousers: nil, shoes: nil)

    let randomCoat = Int(arc4random_uniform(UInt32(allCoats.count)))
    outfitViewController.coatImage.image = UIImage(named: allCoats[randomCoat])
    print(allCoats[randomCoat])

    let randomSweater = Int(arc4random_uniform(UInt32(allSweaters.count)))
    outfitViewController.sweaterImage.image = UIImage(named: allSweaters[randomSweater])
    print(allSweaters[randomSweater])

    let randomTrousers = Int(arc4random_uniform(UInt32(allTrousers.count)))
    outfitViewController.trousersImage.image = UIImage(named: allTrousers[randomTrousers])
    print(allTrousers[randomTrousers])

    let randomShoes = Int(arc4random_uniform(UInt32(allShoes.count)))
    outfitViewController.shoeImage.image = UIImage(named: allShoes[randomShoes])
    print(allShoes[randomShoes])

    //outfitViewController.shoeImage = UIImageView(image: UIImage(named: allShoes[randomShoes]))

    outfitResult = Outfit(coat: allCoats[randomCoat], sweater: allSweaters[randomSweater], trousers: allTrousers[randomTrousers], shoes: allShoes[randomShoes])

    return String(describing: outfitResult)
}