title: Focus on a TextBox in a DataTemplate
body: <p>I have <code>DataTemplate</code> containing a <code>TextBox</code>. I'm setting this template to a listbox item on a selection.</p>
  
  <p>I'm unable to set focus to textbox in the template. I tried to call MyTemplate.FindName, but it ends up with an Invalid Operation Exception: This operation is valid only on elements that have this template applied.</p>
  
  <p>How can I access it?</p>