title: ComboBox itemsSource and default selected value doesn't work
body: <p>I have this code:</p>
  
  <pre><code>&lt;ComboBox Name="cbxWorkers" HorizontalContentAlignment="Right"
            ItemsSource="{Binding Workers}"&gt;
     &lt;ComboBoxItem IsSelected="True" Content="Select" /&gt;
        &lt;ComboBox.ItemTemplate&gt;
           &lt;DataTemplate&gt;
              &lt;ComboBoxItem Content="{Binding LastName}" /&gt;
           &lt;/DataTemplate&gt;
        &lt;/ComboBox.ItemTemplate&gt;
  &lt;/ComboBox&gt;
  </code></pre>
  
  <p>Everything works fine except the second line.
  It gives me a runtime exception:
  Items collection must be empty before using ItemsSource.</p>
  
  <p>How can I deal with that so I will get also all the Workers, and also the item - "Select" as the first item of the combobox?</p>
  
  <p>Thanks a lot :)</p>