title: Error when adding music files to the AudioTrack list - WP8
body: <p>I'm new to on the app development scene, and I'm building a music player for Windows Phone.
  The problem is when I try to import music from the local Music Library to my app.</p>
  
  <p>I have an AudioPlaybackAgent and a List of music that it will play.
  Here's the list:</p>
  
  <pre><code>public static List&lt;AudioTrack&gt; _playList = new List&lt;AudioTrack&gt;
          {
              new AudioTrack(new Uri("Chama os Mulekes.mp3", UriKind.Relative), 
                      "Chama os Mulekes", 
                      "ConeCrewDiretoria", 
                      "Com os Neurônios Evoluindo", 
                      new Uri("https://lh6.googleusercontent.com/-_d2oha1QzNM/UhjYxv3XsLI/AAAAAAAAAWk/QyWgG7ilTAs/s0-d/Cone-Crew-Diretoria-Com-os-neuronios-evoluindo.jpg", UriKind.Absolute))
          };
  </code></pre>
  
  <p>I added a button to the interface that when clicked it imports the local music library files to that playlist. Here's the code:</p>
  
  <pre><code>private async void ImportButton_Click(object sender, RoutedEventArgs e)
          {
              var musicLibrary = await KnownFolders.MusicLibrary.GetFilesAsync();
              foreach (StorageFile _file in musicLibrary)
              {
                  var musicTags = await _file.Properties.GetMusicPropertiesAsync();
                  MusicPlusAgent.AudioPlayer._playList.Add
                      (new AudioTrack (new Uri(_file.Path, UriKind.RelativeOrAbsolute),
                          musicTags.Title,
                          musicTags.Artist,
                          musicTags.Album,
                          null));
              }
          }
  </code></pre>
  
  <p>When run the app and touch the play button, the preset music plays fine, but when I touch the import button the app crashes.
  Any idea on what's wrong here?</p>
  
  <p>Ps. Sorry for the bad english.</p>