swift – 无法将Nil分配给AVCaptureDeviceInput类型

private func captureSetup (position : AVCaptureDevicePosition) {
    var captureError : NSError?
    var captureDevice : AVCaptureDevice!

    for testedDevice in AVCaptureDevice.devicesWithMediaType(AVMediaTypeVideo){
        if (testedDevice.position == position) {
            captureDevice = testedDevice as! AVCaptureDevice
        }
    }

    if (captureDevice == nil) {
        captureDevice = AVCaptureDevice.defaultDeviceWithMediaType(AVMediaTypeVideo)
    }

    var deviceInput : AVCaptureDeviceInput
    do {
        deviceInput = try AVCaptureDeviceInput(device: captureDevicE)
    } catch let error as NSError {
        captureError = error
        deviceInput = nil  // Nil cAnnot be assigned to type AVCaptureDeviceInput
        if captureError != nil { // new if
            print("error: \(captureError?.localizedDescription)")
        }
    }
    captureSession.sessionPreset = AVCaptureSessionPresetHigh

    if (captureError == nil) {
        if (captureSession.canAddInput(deviceInput)) {
            captureSession.addInput(deviceInput)
        }

        self.videoDataOutput = AVCaptureVideoDataOutput()
        self.videoDataOutput!.videoSetTings = [kCVPixelBufferPixelFormatTypeKey: Int(kCVPixelFormatType_32BGRA)]
        self.videoDataOutput!.alwaysDiscardsLateVideoFrames = true
        self.videoDataOutputQueue = dispatch_queue_create("VideoDataOutputQueue",DISPATCH_QUEUE_seriaL)
        self.videoDataOutput!.setSampleBufferDelegate(self,queue: self.videoDataOutputQueue!)

        if (captureSession.canAddOutput(self.videoDataOutput)) {
            captureSession.addOutput(self.videoDataOutput)
        }
    }

    visageCameraView.frame = UIScreen.mainScreen().bounds

    let previewLayer: AVCaptureVideoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession) as AVCaptureVideoPreviewLayer // changed to let
    previewLayer.frame = UIScreen.mainScreen().bounds
    previewLayer.videoGravity = AVLayerVideoGravityResizeAspectFill
    visageCameraView.layer.addSublayer(previewLayer)
}