Swiftで動画を撮りたい

Swiftで動画をとってUIImageViewに表示するプログラムを作成しました。しかしCameraUtilのimageFromSampleBufferの1行目でunexpectedly found nilエラーが出てしまいます。

どのようにすればoutputからUIImageを取得することができるのでしょうか。

ViewController.swift

import UIKit
import AVFoundation

class ViewController: UIViewController, AVCaptureVideoDataOutputSampleBufferDelegate {

    @IBOutlet weak var imageView: UIImageView!

    var session : AVCaptureSession!
    var device: AVCaptureDevice!
    var output: AVCaptureVideoDataOutput!

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        if self.initCamera(){
            self.session.startRunning()
        }
    }

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

    func initCamera() -> Bool{
        self.session = AVCaptureSession()
        self.session.sessionPreset = AVCaptureSessionPresetMedium

        let devices = AVCaptureDevice.devices()

        for device in devices{
            if(device.position == AVCaptureDevicePosition.Back){
                self.device = device as! AVCaptureDevice
            }
        }
        if self.device == nil{
            return false
        }

        let input: AVCaptureDeviceInput
        do{
            input = try AVCaptureDeviceInput(device: self.device) as AVCaptureDeviceInput
        }catch{
            return false
        }

        if self.session.canAddInput(input){
            self.session.addInput(input)
        }else{
            return false
        }

        self.output = AVCaptureVideoDataOutput()
        self.output.videoSettings = [ kCVPixelBufferPixelFormatTypeKey: Int(kCVPixelFormatType_32BGRA)]

        do{
            try self.device.lockForConfiguration()
            self.device.activeVideoMinFrameDuration = CMTimeMake(1, 15)
            self.device.unlockForConfiguration()
        }catch{
            return false
        }

        let queue: dispatch_queue_t = dispatch_queue_create("myqueue", nil)
        self.output.setSampleBufferDelegate(self, queue: queue)

        self.output.alwaysDiscardsLateVideoFrames = true

        if self.session.canAddOutput(self.output){
            self.session.addOutput(self.output)
        }else{
            return false
        }

        for connection in self.output.connections{
            if let conn = connection as? AVCaptureConnection{
                if conn.supportsVideoOrientation{
                    conn.videoOrientation = AVCaptureVideoOrientation.Portrait
                }
            }
        }
        return true
    }

    func captureOutput(captureOutput: AVCaptureOutput!, didDropSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!) {
        dispatch_async(dispatch_get_main_queue(), {
            if sampleBuffer != nil{
               self.imageView.image = CameraUtil.imageFromSampleBuffer(sampleBuffer)
            }else{
                print("sample buffer is nil")
            }

        })
    }


}

CameraUtil.swift

import Foundation
import UIKit
import AVFoundation

class CameraUtil {
    class func imageFromSampleBuffer(sampleBuffer: CMSampleBuffer)->UIImage{
        // サンプルバッファからピクセルバッファを取り出す
        let pixelBuffer:CVImageBufferRef = CMSampleBufferGetImageBuffer(sampleBuffer)!

        // ピクセルバッファをベースにCoreImageのCIImageオブジェクトを作成
        let ciImage = CIImage(CVPixelBuffer: pixelBuffer)

        //CIImageからCGImageを作成
        let pixelBufferWidth = CGFloat(CVPixelBufferGetWidth(pixelBuffer))
        let pixelBufferHeight = CGFloat(CVPixelBufferGetHeight(pixelBuffer))
        let imageRect:CGRect = CGRectMake(0,0,pixelBufferWidth, pixelBufferHeight)
        let ciContext = CIContext.init()
        let cgimage = ciContext.createCGImage(ciImage, fromRect: imageRect )

        // CGImageからUIImageを作成
        let image = UIImage(CGImage: cgimage)
        return image
    }
}

ちなみに以下のサイトを参考にしています。

http://giveitashot.hatenadiary.jp/entry/2014/10/19/190505
http://qiita.com/koki_h/items/91d9bf918df7c5788ffc