ios - NSRangeException - オブザーバーを削除できません

Question

カメラから画像をキャプチャしてビデオを録画しようとしていますが、カメラでビューを閉じるたびに次のエラーが発生します。

キャッチされない例外 'NSRangeException' が原因でアプリを終了しています。理由: 'オブザーバーとして登録されていないため、App.CameraViewController 0x16ea7f70 からキー パス "movieFileOutput.recording" のオブザーバー App.CameraViewController 0x16ea7f70 を削除できません。*** First throw call stack: (0x2a9e6fef 0x3908ac8b 0x2a9e6f35 0x2b6850f5 0x2b684b07 0x2b6849ab 0xbeb40 0xbca88 0x85c173 0x864d67 0x85ea61 0x866b09 0x867e19 0x39782dc1 0x39782b14) libc++abi.dylib: terminating with uncaught exception of type NSException

私のコード:

class CameraViewController: UIViewController, CameraDelegate, AVCaptureFileOutputRecordingDelegate {

    // MARK:
    // MARK: - Property
    @IBOutlet weak var captureButton: UIButton!
    @IBOutlet var previewView: CameraPreviewView!

    weak var cameraDelegate: CameraDelegate?

    var sessionQueue: dispatch_queue_t?
    var captureSession: AVCaptureSession?
    var stillImageOutput: AVCaptureStillImageOutput?
    var videoDeviceInput: AVCaptureDeviceInput?
    var movieFileOutput: AVCaptureMovieFileOutput?

    var previewLayer: AVCaptureVideoPreviewLayer?

    var deviceAuthorized: Bool  = false
    var backgroundRecordId: UIBackgroundTaskIdentifier = UIBackgroundTaskInvalid
    var sessionRuningAndDeviceAuthorized: Bool {
        get {
            return (self.captureSession?.running != nil && self.deviceAuthorized )
        }
    }
    var runtimeErrorHandlingObserver: AnyObject?
...
}

オブザーバーの追加:

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)

        dispatch_async(self.sessionQueue!, {
            // TODO: Make constants
            self.addObserver(self, forKeyPath: "sessionRunningAndDeviceAuthorized", options: NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New, context: &SessionRunningAndDeviceAuthorizedContext)
            self.addObserver(self, forKeyPath: "stillImageOutput.capturingStillImage", options: NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New, context: &CapturingStillImageContext)
            self.addObserver(self, forKeyPath: "movieFileOutput.recording", options: NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New, context: &RecordingContext)


            NSNotificationCenter.defaultCenter().addObserver(self, selector: "subjectAreaDidChange:", name: AVCaptureDeviceSubjectAreaDidChangeNotification, object: self.videoDeviceInput?.device)

            weak var weakSelf = self

            self.runtimeErrorHandlingObserver = NSNotificationCenter.defaultCenter().addObserverForName(AVCaptureSessionRuntimeErrorNotification, object: self.captureSession, queue: nil, usingBlock: {
                (note: NSNotification?) in
                var stronSelf: CameraViewController = weakSelf!
                dispatch_async(stronSelf.sessionQueue!, {
                    if let sess = stronSelf.captureSession {
                        sess.startRunning()
                    }
                })

            })

            self.captureSession?.startRunning()
        })                                    
    }

そして、オブザーバーを削除するコード:

override func viewWillDisappear(animated: Bool) {
        super.viewWillDisappear(animated)

        dispatch_async(self.sessionQueue!, {

            if let sess = self.captureSession {
                sess.stopRunning()

                NSNotificationCenter.defaultCenter().removeObserver(self, name: AVCaptureDeviceSubjectAreaDidChangeNotification, object: self.videoDeviceInput?.device)
                NSNotificationCenter.defaultCenter().removeObserver(self.runtimeErrorHandlingObserver!)

                // TODO: Make constants

                self.removeObserver(self, forKeyPath: "sessionRunningAndDeviceAuthorized", context: &SessionRunningAndDeviceAuthorizedContext)
                self.removeObserver(self, forKeyPath: "stillImageOutput.capturingStillImage", context: &CapturingStillImageContext)
                self.removeObserver(self, forKeyPath: "movieFileOutput.recording", context: &SessionRunningAndDeviceAuthorizedContext)

            }
        })
    }