How to make a playlist for Win 8.1 metro app?

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Windows

C#5.0

Metro-design

Win8

Hello everyone, I am try to make a Win 8.1 Music Player Metro App. The app must have a playlist that shows all the songs in a ListBox. When I try to load a playlist a UnhandledException is thrown. The same with when I try to add a song to the playlist.

Please I really need help.
The code I have been trying is below.

public sealed partial class MainPage : Page
{

    public static readonly string[] audioExtensions = new string[] { ".wma", ".mp3", ".mp2", ".aac", ".adt", ".adts", ".m4a" };
    public static readonly string[] playlistExtensions = new string[] { ".m3u", ".wpl", ".zpl" };
    Playlist playlist = new Playlist();

    public MainPage()
    {
        this.InitializeComponent();

    }

    async private void Page_Loaded(object sender, RoutedEventArgs e)
    {
        try
        {
            playlist = await PickPlaylistAsync();

            if (playlist != null)
            {
                foreach (StorageFile file in playlist.Files)
                {
                    MusicProperties properties = await file.Properties.GetMusicPropertiesAsync();
                    lstPlaylist.Items.Add(properties.Title);
                }
            }
        }
        catch (Exception)
        {
            throw;
        }
    }

    async private void btnAdd_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            FileOpenPicker picker = CreateFilePicker(audioExtensions);
            IReadOnlyList<StorageFile> files = await picker.PickMultipleFilesAsync();

            if (files != null && files.Count > 0)
            {
                foreach (StorageFile file in files)
                {
                    playlist.Files.Add(file);

                }
                await playlist.SaveAsync();
            }
        }
        catch (Exception)
        {
            throw;
        }
    }

    async public static Task<Playlist> PickPlaylistAsync()
    {
        FileOpenPicker picker = CreateFilePicker(playlistExtensions);
        StorageFile file = await picker.PickSingleFileAsync();

        Playlist playlist = null;
        if (file != null)
        {
            return await Playlist.LoadAsync(file);
        }

        return playlist;
    }

    public static FileOpenPicker CreateFilePicker(string[] extensions)
    {
        FileOpenPicker picker = new FileOpenPicker();
        picker.SuggestedStartLocation = PickerLocationId.MusicLibrary;

        foreach (string extension in extensions)
        {
            picker.FileTypeFilter.Add(extension);
        }
        return picker;
    }

    async private void btnSave_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            if (PlaylistName.Text != "")
            {
                StorageFile savedFile = await playlist.SaveAsAsync(KnownFolders.MusicLibrary, PlaylistName.Text, NameCollisionOption.ReplaceExisting, PlaylistFormat.WindowsMedia);
            }
            else
            {

            }

        }
        catch (Exception)
        {
            throw;
        }

    }