title: ItemsPanelTemplate of a ListView throws apparently wrongly an exception
body: <p>I created a user control which contains a ListView with a custom ItemsPanelTemplate.</p>
  
  <pre><code>&lt;UserControl x:Class="..."
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Name="thisUserControl"&gt;
  &lt;ListView ItemsSource="{Binding Source={StaticResource cvs}}"
            Name="mainListView"&gt;
      &lt;ListView.GroupStyle&gt;
          &lt;GroupStyle&gt;
              ...
          &lt;/GroupStyle&gt;
      &lt;/ListView.GroupStyle&gt;
      &lt;ListView.ItemsPanel&gt;
          &lt;ItemsPanelTemplate&gt;
              &lt;cd:TimeLinePanel UnitsPerSecond="{Binding ElementName=thisUserControl,Path=DataContext.UnitsPerSecond}" Start="{Binding ElementName=thisUserControl, Path=DataContext.Start}"/&gt;
          &lt;/ItemsPanelTemplate&gt;
      &lt;/ListView.ItemsPanel&gt;
  &lt;/ListView&gt;
  </code></pre>
  
  <p>The DataContext of the UserControl has two properties Start and UnitsPerSecond. As I am using groups I cannot simply write </p>
  
  <pre><code>Start={Binding Path=.Start}
  </code></pre>
  
  <p>Thus I used the code above. But if I change the binding of Start to this I get an exception:</p>
  
  <p><em>VisualTree of ItemsPanelTemplate must be a single element.</em></p>
  
  <p>Obviously the ItemsPanelTemplate has got just a single element. </p>
  
  <p>So what could be the problem? My custom panel does not create any elements. It just arranges them.</p>