对实际具有值的可选项展开时为零

我发现了一个非常奇怪的错误,而展开一个可选的。当我打印值时,我实际上可以看到它,但是,它崩溃了,它说:

致命错误:在展开可选值时意外找到nil

这是密码( fetchPost() 是在 viewDidLoad() :

func fetchPost() {
    Api.Posts.observePostIncludingImages(withId: postId) { (post, photosStringArray) in

        self.isLiked(postId: post.id!, completion: { boolValue in
            Api.Posts.fetchCount(postId: post.id!, completion: { totalCount in
                post.isLiked = boolValue

                self.photosArrayString = photosStringArray
                print(photosStringArray) // prints the actual image string
                self.setScrollView()

            })
        })
    }
}

func setScrollView() {
    for  i in stride(from: 0, to: photosArrayString.count, by: 1) {
        var frame = CGRect.zero
        frame.origin.x = self.galleryScrollView.frame.size.width * CGFloat(i)
        frame.origin.y = 0
        frame.size = self.galleryScrollView.frame.size
        galleryScrollView.isPagingEnabled = true

        let newUIImageView = UIImageView()
        print(photosArrayString[i]) // Prints the value again
        let myImage = UIImage(named: photosArrayString[i])! // CRASHES! i = 0 (first item in the array)

        avgBackgroundColorsOfImage.append(myImage.averageColor!)

        newUIImageView.image = myImage
        newUIImageView.frame =  frame

        newUIImageView.contentMode = UIViewContentMode.scaleAspectFit

        newUIImageView.sd_setImage(with: URL(string: self.photosArrayString[i]), completed: nil)


        galleryScrollView.backgroundColor = avgBackgroundColorsOfImage[i]
        galleryScrollView.addSubview(newUIImageView)

        self.galleryScrollView.contentSize = CGSize(width: self.galleryScrollView.frame.size.width * CGFloat(photosArrayString.count),
                                                    height: self.galleryScrollView.frame.size.height)
        pageControl.addTarget(self, action: #selector(changePage), for: UIControlEvents.valueChanged)
    }
}