AVAssetReader(asset: asset): The requested URL was not found on this server.

I'm trying the sample app from here:

https://developer.apple.com/documentation/vision/detecting_moving_objects_in_a_video

I made a tweak to read the video from library instead of document picker:

var recordedVideoURL: AVAsset?

@IBAction func uploadVideoForAnalysis(_ sender: Any) {
        var configuration = PHPickerConfiguration()
        configuration.filter = .videos
        configuration.selectionLimit = 1

        let picker = PHPickerViewController(configuration: configuration)
        picker.delegate = self

        present(picker, animated: true, completion: nil)
    }

The delegation method:

extension HomeViewController: PHPickerViewControllerDelegate {
    func picker(_ picker: PHPickerViewController, didFinishPicking results: [PHPickerResult]) {
        guard let selectedResult = results.first else {
            print("assetIdentifier: null")
            dismiss(animated: true, completion: nil)
            return
        }

        selectedResult.itemProvider.loadFileRepresentation(forTypeIdentifier: UTType.movie.identifier) { [weak self] url, error in
            guard error == nil, let url = url else {
                print(error?.localizedDescription ?? "Failed to load image")
                return
            }
            let asset = AVAsset(url: url)
            self?.recordedVideoURL = asset 

            DispatchQueue.main.async { [weak self] in

                self?.dismiss(animated: true) { // dismiss the picker
                    self?.performSegue(withIdentifier: ContentAnalysisViewController.segueDestinationId,
                                       sender: self!)
                    self?.recordedVideoURL = nil
                }
            }
        }
    }
}

everything else is pretty much the same. Then, in the camera controller, it raised an error: "The requested URL was not found on this server." I put a debug break point and it show the error was from the line init AssetReader: let reader = AVAssetReader(asset: asset)

func startReadingAsset(_ asset: AVAsset, reader: AVAssetReader? = nil) {
        
        videoRenderView = VideoRenderView(frame: view.bounds)
        setupVideoOutputView(videoRenderView)
        videoFileReadingQueue.async { [weak self] in
            do {
                guard let track = asset.tracks(withMediaType: .video).first else {
                    throw AppError.videoReadingError(reason: "No video tracks found in the asset.")
                }
                
                let reader = AVAssetReader(asset: asset)
                let settings = [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_420YpCbCr8BiPlanarFullRange]
                let output = AVAssetReaderTrackOutput(track: track, outputSettings: settings)
                if reader.canAdd(output) {
                    reader.add(output)
                } else {
                    throw AppError.videoReadingError(reason: "Couldn't add a track to the asset reader.")
                }
                
                if !reader.startReading() {
                    throw AppError.videoReadingError(reason: "Couldn't start the asset reader.")
                }
...

I tried to create a reader directly in the asset creation block, it worked:

 selectedResult.itemProvider.loadFileRepresentation(forTypeIdentifier: UTType.movie.identifier) { [weak self] url, error in
            guard error == nil, let url = url else {
                print(error?.localizedDescription ?? "Failed to load image")
                return
            }
            let asset = AVAsset(url: url)

            do {
                let reader = try AVAssetReader(asset: asset)
                self?.assetReader = reader
                print("reader: \(reader)")
            }
            catch let e { print("No reader: \(e)") }
...

but if I just move it a little bit to the Dispatch.main.async block, it printed out "No reader: The requested URL was not found on this server.

Therefore, I have to keep an instance of the reader and pass it to the cameraVC.

Can someone please explain why is this happening? What's the logic behind this?