title: Append DataGrid inside of DataGrids RowDetailsTemplate
body: <p>this appears to bind, but rows in Details Grid are empty. Something is off/missing?</p>
  
  <p>I've also tried {Binding SubCustomers}</p>
  
  <p>SubCustomers is a List on parent object. </p>
  
  <p>I am able to bind this way to single Fields such as FirstName etc.. just not the subcollection.. </p>
  
  <pre><code>        &lt;DataGrid.RowDetailsTemplate&gt;
              &lt;DataTemplate&gt;
                  &lt;DataGrid AutoGenerateColumns="True" ItemsSource="{Binding Source=SubCustomers}" /&gt;
              &lt;/DataTemplate&gt;
          &lt;/DataGrid.RowDetailsTemplate&gt;
  </code></pre>