致命错误，因为在展开包装时意外发现nil

Question

我目前正在将AVFoundation集成到我的Swift代码中。但是，我无法正确运行该应用程序，因为可选值是nil，即使我已经为此变量设置了一个值。

错误发生在func setupInputOutput()，其中设备currentCamera触发错误。

import UIKit
import AVFoundation
class ViewController: UIViewController {

    var captureSession = AVCaptureSession()
    var backCamera: AVCaptureDevice?
    var frontCamera: AVCaptureDevice?
    var currentCamera: AVCaptureDevice?

    var photoOutput: AVCapturePhotoOutput?

    override func viewDidLoad() {
        super.viewDidLoad()

        setupCaptureSession()
        setupDevice()
        setupInputOutput()
        setupPreviewLayer()
        startRunningCaptureSession()

        // Do any additional setup after loading the view, typically from a nib.
    }

    func setupCaptureSession() {
        captureSession.sessionPreset = AVCaptureSession.Preset.photo
    }

    func setupDevice() {
        let deviceDiscoverySession = AVCaptureDevice.DiscoverySession(deviceTypes: [AVCaptureDevice.DeviceType.builtInWideAngleCamera], mediaType: AVMediaType.video, position: AVCaptureDevice.Position.unspecified)
        let devices = deviceDiscoverySession.devices

        for device in devices {
            if device.position == AVCaptureDevice.Position.back {
                backCamera = device
            } else if device.position == AVCaptureDevice.Position.front {
                frontCamera = device
            }
        }

        currentCamera = backCamera
    }

    func setupInputOutput() {
        do {
            let captureDeviceInput = try AVCaptureDeviceInput(device: currentCamera!)
            captureSession.addInput(captureDeviceInput)
            photoOutput = AVCapturePhotoOutput()
            photoOutput?.setPreparedPhotoSettingsArray([AVCapturePhotoSettings(format: [AVVideoCodecKey: AVVideoCodecType.jpeg])], completionHandler: nil)
            captureSession.addOutput(photoOutput!)
        } catch {
            print(error)
        }
    }

    func setupPreviewLayer() {
    }

    func startRunningCaptureSession() {
    }            

    @IBAction func cameraButton_TouchUpInside(_ sender: Any) {
        performSegue(withIdentifier: "showPhoto_Segue", sender: nil)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }    
}