Focus textbox after async call -

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!

"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(_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");
}
}