title: Add a warning icon to the TextBox when validation
body: <p>I want to validate a textbox if something is wrong. The idea is if something is wrong than the next TextBox should have a warning image.</p>
  
  <pre><code>&lt;TextBox Text="{Binding Age, UpdateSourceTrigger=PropertyChanged}"&gt;
      &lt;Validation.ErrorTemplate&gt;
          &lt;ControlTemplate&gt;
              &lt;StackPanel&gt;
                  &lt;!-- Placeholder for the TextBox itself --&gt;
                  &lt;AdornedElementPlaceholder x:Name="textBox"/&gt;
                  &lt;image source="some-Image.png" width="20" Height="20" /&gt;
              &lt;/StackPanel&gt;
          &lt;/ControlTemplate&gt;
      &lt;/Validation.ErrorTemplate&gt;
  &lt;/TextBox&gt;
  </code></pre>
  
  <p>But the thing is the image is not showing, it only shows the border of the icon.
  Am I using <code>AdornedElementPlaceholder</code> correctly?</p>