Focus textbox after async call -

Question

WPFHi, I have a strange problem. After using an asynchronous call, I can not do focus to a particular control (textbox in this example). If I comment the lines where asynchronous calls, the focus works perfectly. i'm use MVVM Light 5.3.0 Why? Thx!

<TextBox Grid.Row="0"
         Grid.Column="1"
         utilidades:FocusExtension.IsFocused="{Binding IsFocused}"
         Text="{Binding Articulo.Codigo, UpdateSourceTrigger=PropertyChanged}" />

private async void CargarRubros()
{
    PrepareIsBusy();

    ShowIsBusyMessage = true;

    ActiveProgressRing = true;

    IsBusyCustomMessage("Cargando rubros...");

    try
    {
        Rubros =
            await
                Task.Factory.StartNew(() => new ObservableCollection<Rubro>(_rubroService.TodosLosSubrubros()));

        ActiveProgressRing = false;

        if (Rubros.Any())
        {
            RubroSeleccionado = Rubros.First();
        }
        else
        {
            IsBusyInfoMessage("No se encontraron rubros.");

            await Task.Factory.StartNew(() => Thread.Sleep(2000));
        }
    }
    catch (Exception)
    {
        IsBusyErrorMessage("Error al intentar obtener los rubros.");

        ActiveProgressRing = false;

        await Task.Factory.StartNew(() => Thread.Sleep(2000));
    }
    finally
    {
        ShowIsBusyMessage = false;

        ResetMessageAndImage();

        PrepareIsBusy();

        IsFocused = true;
    }
}

public static class FocusExtension
{
    public static bool GetIsFocused(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsFocusedProperty);
    }

    public static void SetIsFocused(DependencyObject obj, bool value)
    {
        obj.SetValue(IsFocusedProperty, value);
    }

    public static readonly DependencyProperty IsFocusedProperty =
        DependencyProperty.RegisterAttached(
            "IsFocused", typeof(bool), typeof(FocusExtension),
            new UIPropertyMetadata(false, null, OnCoerceValue));

    private static object OnCoerceValue(DependencyObject d, object baseValue)
    {
        if ((bool) baseValue)
        {
            ((UIElement) d).Focus();
        }
        else if (((UIElement) d).IsFocused)
        {
            Keyboard.ClearFocus();
        }
        return (bool)baseValue;
    }
}

private bool _isFocused;

public bool IsFocused
{
    get { return _isFocused; }
    set
    {
        _isFocused = value;
        RaisePropertyChanged("IsFocused");
    }
}