title: Event is null even object is not disposed
body: <p>When I call <code>ShowRevertInventorySignDialogView()</code> through COMMAND, event throws ObjectNullException. I'm thinking that Object is not dispose as its assined to <code>CurrentViewModel</code> property.</p>
  
  <p>Both <code>GetReport()</code> and <code>ShowRevertInventorySignDialogView()</code> functions called by Command which are binded to Buttons. I'm pretty sure that <code>GetReport()</code> function is called first so that object is created before raising the event.</p>
  
  <p>What am I missing here?</p>
  
  <pre><code>class MainWindowViewModel : ViewModel{
  
      public ViewModel CurrentViewModel
      {
          get { return currentViewModel; }
          set { currentViewModel = value; NotifyPropertyChanged(); }
      }
  
      public void GetReport()
      {
          inventoryReportViewModel = new InventoryReportViewModel();
          inventoryReportViewModel.OnStatusChange += Event_OnStatusChange;
          CurrentViewModel = inventoryReportViewModel;          
      }
  }
  
  public class InventoryReportViewModel : InventoryBaseViewModel
  {
      public event EventHandler&lt;StatusChangeEventArgs&gt; OnStatusChange;
  
      private void ShowRevertInventorySignDialogView()
      {
          OnStatusChange(this, new StatusChangeEventArgs("test",10));
          ....
          ....
      }
  }
  </code></pre>
  
  <p>XAML;</p>
  
  <pre><code>&lt;ContentControl Content="{Binding Path=CurrentViewModel}" Margin="20 10 20 0"&gt;&lt;/ContentControl&gt;
  </code></pre>