In case you haven't found the solution on the other answers, that's how I solved the issue.
Application.Current.Dispatcher.BeginInvoke(new Action(() =>
{
TEXTBOX_OBJECT.Focus();
}), System.Windows.Threading.DispatcherPriority.Render);
From what I understand the other solutions may not work because the call to Focus()
is invoked before the application has rendered the other components.