title: Caliburn.Micro rebind ContentControl on navigation GoBack
body: <p>I'm using Caliburn.Micro within WinRT application</p>
  
  <p>Here is my main VM:</p>
  
  <pre><code>public class MainViewModel : Conductor&lt;Screen&gt;
  {
      protected override void OnActivate()
      {
          if (ActiveItem == null)
          {
             ActivateItem(
                 ViewModelLocator.LocateForViewType(typeof(NewsFeedView)) as Screen);
          }
  
          base.OnActivate();
      }
  }
  </code></pre>
  
  <p>here I use conductor because I want to load different controls in ContentControl, but now I have only this code. Here is my content control in main view:</p>
  
  <pre><code>&lt;ContentControl x:Name="ActiveItem" Grid.Column="1" Grid.Row="1" /&gt;
  </code></pre>
  
  <p>When I running the application everything work fine, <code>MainViewModel.Activate</code> gets called and <code>ActiveItem</code> set to <code>NewsFeedViewModel</code> and <code>ContentControl</code> loads <code>NewsFeedView</code>.</p>
  
  <p><strong>The problem:</strong></p>
  
  <p>When I navigate in <code>NewsFeedView</code> control to another view using <code>NavigationService.NavigateToViewModel</code> method and then in that view use <code>NavigationService.GoBack</code>, i'm returning to <code>MainView</code> and in that moment when <code>MainViewModel.Activate</code> gets called <code>ActiveItem</code> is not <code>null</code>, but <code>ContentControl.Content</code> is <code>null</code>. I've tried use <code>View.Model</code> attached property for <code>ContentControl</code> but no luck, how to make it rebind?</p>
  
  <p><strong>EDIT:</strong>
  Finally i'm setup logger in Caliburn to see what happens and I found an error - when MainView loaded after navigationg back this events occuring:</p>
  
  <pre><code>Attaching ***.Views.MainView to ***.ViewModels.MainViewModel.
  ViewModel bound on ActiveItem.
  Using cached view for ***.ViewModels.NewsFeedViewModel.
  System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---&gt; System.Exception: Unspecified error
  at Windows.UI.Xaml.Controls.ContentControl.put_Content(Object value)
  ... some winRT stack
  at Caliburn.Micro.View.SetContentPropertyCore(...
  </code></pre>
  
  <p>Though it was not so informative I've used InteliTrace to get more info and got this message: "Element is already child of another element". I suppose NewsFeedView stored somewhere and when time comes to put it in ContentControl this exception thrown. 
  How to solve this?</p>