Crash on older device when app enters background while converting

[tapgitk]

The lib crashed at self.group.leave() in the code block

outputVideoData(videoInput, videoOutput: videoOutput, duration: duration, progressHandler: progressHandler) {
      self.group.leave()
  }

I tested on an iphone 6 ios 12 with a video of size 200mb and after 5 seconds since the conversion started, I minimize the app for 3 seconds and go back to active state. Crash doesnt happen on iphone x with similar reproduction steps. I guess there may be an unparalleled match between group.enter() and group.leave() in the block. Please advise any possible improvement.