title: Win 8.1 ComboBox Display the "ToString" Function Result when the SelectedItem is Null or Empty String
body: <p>I have an XAML Combobox like this:</p>
  
  <pre><code>&lt;ComboBox x:Name="ComboBox" ItemsSource="{Binding Items}"/&gt;
  </code></pre>
  
  <p>And the bond data items are defined below (the <code>DefaultViewModel</code> is a <code>ObservableDictionary</code> object used for data binding):</p>
  
  <pre><code>DefaultViewModel["Items"] = new[] {"This", "That", null};
  </code></pre>
  
  <p>The data binding is working fine. When I click the "This" or "That" options, the selected values are displayed correctly; however when I click the empty option (namely the <code>null</code>), the <code>ToString()</code> result of the bound object (<code>TestApp.Common.ObservableDictionary</code>) is shown instead of just being empty. </p>
  
  <p>What is going wrong here? Thanks!</p>