title: WPF CollectionViewSource sorting issues with null values
body: <p>I've got a collectionviewsource whose source is an observable collection and i'm sorting like so ; </p>
  
  <pre><code>_viewSource.SortDescriptions.Add(new SortDescription() { PropertyName ="PropertyName", Direction = ListSortDirection.Ascending });
  </code></pre>
  
  <p>this all works great until I attempt to sort by a property in the list that has a null value. I then get a InvalidOperationException, "Failed to compare two elements in the array"</p>
  
  <p>Do I have to implement my own IComparer class in order to work around the null issue or am I missing a trick?</p>
  
  <p>Thanks in advance..</p>