[wip] trying to generate thumbnail using AVAssetImageGenerator but not working.

ViewController.swift

//
//  ViewController.swift
//  tvOS11HLSSample
//
//  Created by 鈴木 俊裕 on 2017/06/06.
//  Copyright © 2017 Toshihiro Suzuki. All rights reserved.
//

import UIKit
import AVKit

class ViewController: AVPlayerViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        navigationController?.isNavigationBarHidden = true
        player = AVPlayer(url: URL(string: "http://172.30.20.88/fps-experiment/prog_index.m3u8")!)
        player!.play()
    }
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        navigationController?.isNavigationBarHidden = false
    }
}

class PlayerView: UIView {
    override class var layerClass: AnyClass {
        return AVPlayerLayer.self
    }
    var player:AVPlayer {
        get {
            let layer: AVPlayerLayer = self.layer as! AVPlayerLayer
            return layer.player!
        }
        set {
            let layer: AVPlayerLayer = self.layer as! AVPlayerLayer
            layer.player = newValue
        }
    }
}

class CustomViewController: UIViewController {
    @IBOutlet var durationLabel: UILabel!
    @IBOutlet var progressLabel: UILabel!
    @IBOutlet var slider: UISlider!
    @IBOutlet private weak var playerView: PlayerView!
    private var generator: AVAssetImageGenerator!
    @IBAction func close(_ sender: Any) {
        dismiss(animated: true, completion: nil)
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        navigationController?.isNavigationBarHidden = true
        let asset = AVURLAsset(url: URL(string:"http://172.30.20.88/fps-experiment/prog_index.m3u8")!)
        asset.loadValuesAsynchronously(forKeys: ["playable"]) {
            if asset.isPlayable {
                DispatchQueue.main.async {
                    let item = AVPlayerItem(asset: asset)
                    self.playerView.player = AVPlayer(playerItem: item)
                    self.playerView.player.play()
                    let timescale = item.currentTime().timescale
//                    let times: [NSValue] = (0..<Int(asset.duration.seconds)).map { NSValue(time: CMTime(seconds: Double($0), preferredTimescale: timescale)) }
                    let times = [NSValue(time: CMTime.init(seconds: 50, preferredTimescale: item.currentTime().timescale))]
                    self.generator = AVAssetImageGenerator(asset: asset)
                    self.generator.maximumSize = self.view.frame.size
                    self.generator.generateCGImagesAsynchronously(forTimes: times) { (requestedTime, image, actualTime, result, error) in
                        print(requestedTime)
                        print(image)
                        print(actualTime)
                        print(result)
                        print(error)
                    }
                }
            }
        }
    }
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        navigationController?.isNavigationBarHidden = false
    }
}