convert image selected from gallery to gray scale in iOS

Question

I am writing a program to convert an image selected from photo library to gray scale in iOS, but when I tested it, the imageView show nothing but darkness. What's wrong with my code?

Here is a part of my code. And I'm pretty sure that there is nothing wrong with my toGray function, which I use to convert an image to gray scale by Open CV.

func didSelectPhoto(_ image: UIImage?) {
    if let image = image {
        self.setupElementsVisibility(true)
        let gray =  OpenCVWrapper.toGray(image)
        self.imageContainer.image = gray

    } else {
        self.setupElementsVisibility(true)
    }
    self.dismiss(animated: true, completion: nil)
}