Empty movie file when combining videos in Swift

Question

I'm new to Swift. I'm trying to combine several mp4 files into a single movie, but my code ends up writing an empty video file.

This is the code I wrote:

import Foundation
import AVFoundation

var folderURL = URL(fileURLWithPath: "/")
var outputMovieURL = URL(fileURLWithPath:"/test.mov")


var mp4URLs = try! FileManager
    .default
    .contentsOfDirectory(at: folderURL, includingPropertiesForKeys: nil, options: [.skipsHiddenFiles])
    .filter { forMP4(url:$0) }
        
func forMP4(url: URL) -> Bool {
    return url.pathExtension == "mp4"
}


let movie = AVMutableComposition()
let videoTrack = movie.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)


for url in mp4URLs {
    if #available(macOS 12.0, *) {
        do {
                let videoAsset = AVAsset(url: url)
                let firstVideoTrack = try await videoAsset.loadTracks(withMediaType: .video).first
                let range = CMTimeRangeMake(start: CMTime.zero, duration: videoAsset.duration)
                try videoTrack?.insertTimeRange(range, of: firstVideoTrack!, at: CMTime.zero)
            } catch {
                print("Failed to insert video data from URL: \(url)")
                print("Error: \(error.localizedDescription)")
            }
        print(movie.duration)
    } else {
        // Fallback on earlier versions
    }
}

let exporter = AVAssetExportSession(asset: movie,
                               presetName: AVAssetExportPresetHighestQuality)
exporter?.outputURL = outputMovieURL
exporter?.outputFileType = .mov
//export!
exporter?.exportAsynchronously(completionHandler: { [weak exporter] in
  DispatchQueue.main.async {
    if let error = exporter?.error {
      print("failed \(error.localizedDescription)")
    } else {
      print("movie has been exported to \(outputMovieURL)")
    }
  }
})

What is odd is that the duration of it increments as I include videos. This is the output of the code's print(movie.duration) command.

CMTime(value: 1135671, timescale: 1000, flags: __C.CMTimeFlags(rawValue: 1), epoch: 0)
CMTime(value: 1153202, timescale: 1000, flags: __C.CMTimeFlags(rawValue: 1), epoch: 0)
CMTime(value: 1801843, timescale: 1000, flags: __C.CMTimeFlags(rawValue: 1), epoch: 0)
CMTime(value: 1944314, timescale: 1000, flags: __C.CMTimeFlags(rawValue: 1), epoch: 0)