title: Validation error template - binding exception
body: <p>I am using the following template for <strong>validation error</strong>:</p>
  
  <pre><code>&lt;ControlTemplate&gt;
      &lt;Border BorderBrush="Red" BorderThickness="1"&gt;
          &lt;Grid&gt;
              &lt;Polygon Points="8,8 8,0 0,0"
                       Stroke="Black"
                       StrokeThickness="1"
                       Fill="Red"
                       HorizontalAlignment="Right"
                       VerticalAlignment="Top"
                       ToolTip="{Binding ElementName=adorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}" /&gt;
              &lt;AdornedElementPlaceholder x:Name="adorner"/&gt;
          &lt;/Grid&gt;
      &lt;/Border&gt;
  &lt;/ControlTemplate&gt;
  </code></pre>
  
  <p>ToolTip is working fine, but after leaving the current record the following exception is thrown:</p>
  
  <blockquote>
    <p>System.Windows.Data Error: 17 : Cannot get 'Item[]' value (type
    'ValidationError') from '(Validation.Errors)' (type
    'ReadOnlyObservableCollection`1').
    BindingExpression:Path=AdornedElement.(0)[0].ErrorContent;
    DataItem='AdornedElementPlaceholder' (Name='adorner'); target element
    is 'Polygon' (Name=''); target property is 'ToolTip' (type 'Object')
    ArgumentOutOfRangeException:'System.ArgumentOutOfRangeException:
    Specified argument was out of the range of valid values.</p>
  </blockquote>
  
  <p>I tried with <code>HasError</code> property, but failed. Anyone has any idea?</p>