title: [NSMutableArray objectAtIndex:]: index 0 beyond bounds for empty array from AVURLAsset
body: <p>I tried with following code to merge the audios. </p>
  
  <pre><code>AVMutableComposition* composition = [AVMutableComposition composition];
  AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:audioURL options:nil];
  AVURLAsset* audioAsset1 = [[AVURLAsset alloc]initWithURL:audioURL1 options:nil];
  AVURLAsset* audioAsset2 = [[AVURLAsset alloc]initWithURL:audioURL1 options:nil];
  AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
  NSError* error = NULL;
  [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,videoAsset.duration) 
                                         ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo]objectAtIndex:0] 
                                          atTime:kCMTimeZero  
  AVMutableCompositionTrack *compositionAudioTrack1 = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
          [compositionAudioTrack1 insertTimeRange:CMTimeRangeMake(kCMTimeZero,audioAsset1.duration) 
                                         ofTrack:[[audioAsset1 tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0] 
                                          atTime:kCMTimeZero
                                           error:&error];
  AVMutableCompositionTrack *compositionAudioTrack2 = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
          [compositionAudioTrack2 insertTimeRange:CMTimeRangeMake(kCMTimeZero,audioAsset2.duration) 
                                         ofTrack:[[audioAsset2 tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0] 
                                          atTime:kCMTimeZero
                                           error:&error];
  </code></pre>
  
  <p>When I try to execute the "[[audioAsset1 tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0]" it is giving following error</p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSRangeException', 
  reason: '*** -[NSMutableArray objectAtIndex:]: index 0 beyond bounds for empty array'</code></pre>
  
  <p>But when I try to print the audioAsset1 in console, it is printing "&lt;AVURLAsset: 0x198920, URL = 'path'&gt;". I have checked with the path, that file is there. </p>
  
  <p>what is my mistake? Please help me to resolve it.</p>