data of image from url is nil after unwrapping

Question

I'm using Swift 4 and trying to get images from my server. Using ipconfig in terminal, I know my localhost's ip is 10.43.229.215. the following code is to retrieve the image data and turn it into UIImage:

func GetImage(url:String) {
        let image = String(localIP + url).addingPercentEncoding(withAllowedCharacters: .urlQueryAllowed)!
        let url = URL(string: image)
        print(image)
        URLSession.shared.dataTask(with: url!, completionHandler: { (data, response, error) in
            if error != nil {
                print("Client error!")
                return
            }
            guard let data = data else {
                print("Error: did not receive data")
                return
            }
            DispatchQueue.main.async {
                self.LensImageView.image = UIImage(data: data)
            }
        }).resume()
    }

What I don't understand is that, the image string did show the image I want if I copy/paste the string to my browser (http://10.43.229.215:3000/lensPic/%E5%A4%AA%E5%A6%83%E7%B3%96%E6%9D%8F.png) However, error appears at the line self.LensImageView.image = UIImage(data: data) saying Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value. I'm really confused about:

1. How can data be nil if there is already a guard let method?
2. Why can the data be nil is I can show the image through my browser?

Any help is highly appreciated!

Answer 1

The problem lies in making a false assumption about what is nil. It has nothing to do with the image or the data. What’s nil is self.LensImageView, the outlet property.