title: Clear ComboBox with attached ItemsSource? -> ItemsSource = null -> NullReferenceException
body: <p>I have a <code>CheckBox chkAddToGroup</code> which activates this <code>ComboBox cmbGroup</code>.
  If <code>chkAddToGroup</code> is being checked the following happens:</p>
  
  <pre><code>ObservableCollection&lt;Group&gt; groupColl = new ObservableCollection&lt;Group&gt;() { };
  foreach (Group g in GroupHandler.GroupList)
  {
      groupColl.Add(g);
  }
  cmbGroup.ItemsSource = groupColl;
  cmbOrganisation.SelectedIndex = 0;
  cmbGroup.IsEnabled = true;
  </code></pre>
  
  <p>As you can see <code>cmbGroup</code> is attached to <code>groupColl</code> (<code>DisplayMemberPath</code> is set in an external <code>Style</code>).</p>
  
  <p>The problem occurs if I try to reset my form. Therefor I have a <code>Button btnReset</code>.
  If the user clicks on <code>btnReset</code> the application does this:</p>
  
  <pre><code>chkAddToGroup_Unchecked(this, null);
  </code></pre>
  
  <p>In <code>chkAddToGroup_Unchecked(object sender, RoutedEventArgs e)</code> following happens then:</p>
  
  <pre><code>private void chkAddToGroup_Unchecked(object sender, RoutedEventArgs e)
      {
          cmbGroup.ItemsSource = null;
          cmbGroup.IsEnabled = false;
          cmbGroupRole.ItemsSource = null;
          cmbGroupRole.IsEnabled = false;
      }
  </code></pre>
  
  <p>As you can see I set <code>ItemsSource = null</code> in order to clear the combobox. (don't know any other way atm)</p>
  
  <p>If the user didn't check the checkbox before hitting the reset button then nothing special happens.
  But if the user did check the checkbox and selected an item then there is a <code>NullReferenceException</code> as soon as the application tries to execute <code>cmbGroup.ItemsSource = null;</code></p>
  
  <p>Why does this happen? Any idea?</p>