private void GoToPlayer_Click(object sender, RoutedEventArgs e)
    {

        if (Directory.Exists(this.main.workingDir + "/" + IDText.Text + "/Tracks")) // Error occurs here
        {
            this.main.Content = this.main.player; //here i switch between different ui forms
        }
        else
        {
            MessageBox.Text = "CD not found";
            IDText.Text = "";
        }

    }

An exception of type 'System.InvalidOperationException' occurred in System.IO.FileSystem.dll but was not handled in user code

private async void GoToPlayer_Click(object sender, RoutedEventArgs e)
    {
        await this.Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => {
            if (Directory.Exists(this.main.workingDir + "/" + IDText.Text + "/Tracks")) // Error occurs here
            {
                this.main.Content = this.main.player; //here i switch between different ui forms
            }
            else
            {
                MessageBox.Text = "CD not found";
                IDText.Text = "";
            }
        });
    }

private async void GoToPlayer_Click(object sender, RoutedEventArgs e)
{
    var folder = await StorageFolder.GetFolderFromPathAsync(main.workingDir);

    if ((folder = await folder.TryGetItemAsync(IDText.Text) as StorageFolder) != null &&
        (folder = await folder.TryGetItemAsync("Tracks") as StorageFolder) != null)
    {
        ...
    }
}

var picker = new FolderPicker();
picker.SuggestedStartLocation = PickerLocationId.MusicLibrary;
picker.FileTypeFilter.Add(".mp3");
var folder = await picker.PickSingleFolderAsync();
var result = await Task.Run(() => Directory.Exists(Path.Combine(folder.Path, "foobar")));
if (result)
{
  Debug.WriteLine("Yes");
}
else
{
  Debug.WriteLine("No");
}