title: subscribing to property changes in View throws ArgumentNullException
body: <p>I have code that looks similar to the following</p>
  
  <p>ViewModel:</p>
  
  <pre><code>public class VM : ReactiveObject
  {
      public double _number;
      public double Number
      {
          get { return _number; }
          set { this.RaiseAndSetIfChanged(ref _number, value); }
      }
  }
  </code></pre>
  
  <p>View:</p>
  
  <pre><code>public sealed partial class MainPage : Page, IViewFor&lt;VM&gt;
  {
      public MainPage()
      {
          ViewModel = new VM();
          DataContext = ViewModel;
          this.InitializeComponent();
  
          this.WhenAnyValue(t =&gt; t.ViewModel.Number)
              .Subscribe(n =&gt; Debug.WriteLine(n));
      }
  
      public VM ViewModel
      {
          get { return (VM)GetValue(ViewModelProperty); }
          set { SetValue(ViewModelProperty, value); }
      }
  
      public static readonly DependencyProperty ViewModelProperty =
          DependencyProperty.Register("ViewModel", typeof(VM), typeof(MainPage), new PropertyMetadata(null));
  
      object IViewFor.ViewModel
      {
          get { return ViewModel; }
          set { ViewModel = (VM)value; }
      }
  
      private void Button_Click(object sender, RoutedEventArgs e)
      {
          ViewModel.Number = ViewModel.Number + 1;
      }
  }
  </code></pre>
  
  <p>When I run it I get an ArgumentNullException on the "WhenAnyValue" line.</p>
  
  <p>This seems to follow every example of usage I can find.  I'm at a loss here.</p>