java - mp4parser AppendTrack エラー java.lang.NullPointerException

Question

これはエラーです：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.googlecode.mp4parser.boxes.mp4.ESDescriptorBox.getContentSize(ESDescriptorBox.java:60)
    at com.googlecode.mp4parser.AbstractBox.getSize(AbstractBox.java:203)
    at com.googlecode.mp4parser.BasicContainer.getContainerSize(BasicContainer.java:70)
    at com.coremedia.iso.boxes.sampleentry.AudioSampleEntry.getSize(AudioSampleEntry.java:330)
    at com.googlecode.mp4parser.BasicContainer.getContainerSize(BasicContainer.java:70)
    at com.coremedia.iso.boxes.SampleDescriptionBox.getSize(SampleDescriptionBox.java:112)
    at com.googlecode.mp4parser.AbstractContainerBox.getHeader(AbstractContainerBox.java:67)
    at com.coremedia.iso.boxes.SampleDescriptionBox.getBox(SampleDescriptionBox.java:94)
    at com.googlecode.mp4parser.authoring.tracks.AppendTrack.mergeStsds(AppendTrack.java:81)
    at com.googlecode.mp4parser.authoring.tracks.AppendTrack.<init>(AppendTrack.java:65)
    at Mp4.merging.merge(merging.java:97)

そしてこの私のコード：

 if (audioTracks.size() > 0) {
            result.addTrack(new AppendTrack(audioTracks.toArray(new Track[audioTracks.size()])));
        }

そして、 Null Element が見つかりません。