CollectionViewController

Question

我不知道处理CollectionView的代码有什么问题。编译器显示一个错误，指出以NSException类型的未捕获异常终止。另一个TableViewController与相同的数据模型一起工作正常。当我单击集合视图的选项卡栏时，它会抛出错误。

import Foundation
import UIKit

class MemeCollectionViewController: UICollectionViewController {
var memes: [Meme]!
@IBOutlet var memeCollectionView: UICollectionView!

override func viewDidLoad() {
    super.viewDidLoad()
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    memes = appDelegate.memes
    createRightBarButtonItem()

}

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(true)
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
    memes = appDelegate.memes

}

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(true)
    memeCollectionView.reloadData()
}

func createRightBarButtonItem() {
    navigationItem.rightBarButtonItem = UIBarButtonItem(
        title: "+",
        style: .plain,
        target: self,
        action: #selector(goToViewController))
}

func goToViewController(){
    let createMemeViewController = self.storyboard!.instantiateViewController(withIdentifier: "CreateMemeViewController") as! CreateMemeViewController
    present(createMemeViewController, animated: true, completion: nil)
}

override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return memes.count
}

override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "MemeCollectionViewCell", for: indexPath) as! MemeCollectionViewCell
    let meme = self.memes[(indexPath as NSIndexPath).row]
    cell.memeCollectionImageView?.image = meme.memedImage

    return cell
}

}