iOS Swift: video thumbnail error

问题:

I'm creating video thumbnails with the following code and it works in most cases. However sometimes it throws an error and the thumbnail is not created. I can't figure out why it does not work sometimes.

Here is how I create video thumbnail:

let asset = AVAsset(URL: url)
let imageGenerator = AVAssetImageGenerator(asset: asset)
imageGenerator.appliesPreferredTrackTransform = true

do {
    let cgImage = try imgGenerator.copyCGImageAtTime(CMTimeMake(1, 30), actualTime: nil)
    let uiImage = UIImage(CGImage: cgImage)
    imageview.image = uiImage  
}
catch let error as NSError
{
    print("Image generation failed with error \(error)")
}

It works in most cases, when it does not work it throws this error:

Error generating thumbnail: Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSLocalizedDescription=The operation could not be completed, NSUnderlyingError=0x17658520 {Error Domain=NSOSStatusErrorDomain Code=-12792 "(null)"}, NSLocalizedFailureReason=An unknown error occurred (-12792)}

I have no clue why it throws this error and did not find any solution on stack overflow even though other people faced same problem.