title: ListPicker "SelectedItem must always be set to a valid value"
body: <p>I try to clear (and add) items to <code>ListPicker</code>, but when the app have to clear all of the items is an error - "SelectedItem must always be set to a valid value". My code:</p>
  
  <pre><code>&lt;toolkit:ListPicker x:Name="select" HorizontalAlignment="Right" Margin="0,135,30,0" VerticalAlignment="Top" Width="195" Height="64" d:LayoutOverrides="HorizontalAlignment" BorderBrush="{x:Null}" FontFamily="Arial" FontWeight="Bold" FontSize="32" Style="{StaticResource ListPickerStyle1}" BorderThickness="0" DataContext="{Binding}"&gt;
      &lt;toolkit:ListPicker.Background&gt;
          &lt;ImageBrush Stretch="Fill" ImageSource="list_picker.png"/&gt;
      &lt;/toolkit:ListPicker.Background&gt;
  </code></pre>
  
  <p>and action for a button</p>
  
  <pre><code>private void button_Tap(object sender, System.Windows.Input.GestureEventArgs e)
  {
      select.Items.Clear(); //here is an error
      for (int i = 0; i &lt; arrays.Length; i++)
      {
          select.Items.Add(arrays[i]);
      }
  }
  </code></pre>
  
  <p>I try another options, but it doesn't work too.</p>
  
  <pre><code>private void button_Tap(object sender, System.Windows.Input.GestureEventArgs e)
  select.SelectedItem = null; // here is an error
      select.Items.Clear();
      for (int i = 0; i &lt; arrays.Length; i++)
      {
          select.Items.Add(arrays[i]);
      }
  }
  </code></pre>