title: PropertyChanged is null when binding to static singleton property
body: <pre><code>&lt;Grid DataContext="{StaticResource BaseViewModel}"&gt;
          &lt;DockPanel&gt;
              &lt;TabContro&gt;
                  &lt;TabItem DataContext="{StaticResource MainWindowsViewModel}"&gt;
                      &lt;Tabs:Main /&gt;
                  &lt;/TabItem&gt;
              &lt;/TabControl&gt;
          &lt;/DockPanel&gt;
          &lt;Popup IsOpen="{Binding Path=Common.IsLoading}"&gt;
              &lt;Control Style="{StaticResource BusyAnimationStyle}"/&gt;
          &lt;/Popup&gt;
  &lt;/Grid&gt;
  </code></pre>
  
  <p>And:</p>
  
  <pre><code>public class Common : NotificationObject
  {
      private static Common _instance = null;
  
      private bool _isLoading;
      public bool IsLoading
      {
          get { return _isLoading; }
          set
          {
              if (_isLoading != value)
              {
                  _isLoading = value;
                  RaisePropertyChanged(() =&gt; IsLoading);
              }
          }
      }
  
      protected Common()
      {
  
      }
  
      public static Common GetInstance()
      {
          if (_instance == null)
              _instance = new Common();
  
          return _instance;
      }
  }
  </code></pre>
  
  <p>There is 2 ViewModels
  BaseViewModel
  MainWindowsViewModel</p>
  
  <p>the BaseViewModel is the father of MainWindow
  and BaseViewModel have the common as property</p>
  
  <p>the problem is when MainWindowVM change IsLoading property its not effect
  the PropertyChanged is null
  Any Solution?</p>