title: "'MainViewModel' type must derive from DependencyObject"
body: <p>I'm trying to reconfigure my MVVMLight SL5 app for MVVMLight WPF.<br>
  I think I have hooked up all the correct GalaSoft libs, but on startup I get the exception shown in the title.<br>
  It occurs in ViewModelLocator when getting the ViewModel for MainPage:  </p>
  
  <pre><code>    public MainViewModel Main
      {
          get
          {
              MainViewModel mvm = null;
              try
              {
                  mvm = ServiceLocator.Current.GetInstance&lt;MainViewModel&gt;();
              }
              catch (Exception xx)
              {
                  System.Diagnostics.Debug.WriteLine(xx.Message);
              }
              return mvm;
          }
      }
  </code></pre>
  
  <p>The exception comes from the GetInstance() call.<br>
  MainPage is derived from Window; DataContext is set in the xaml.  </p>
  
  <pre><code>    DataContext="{Binding Main, Source={StaticResource Locator}}"
  </code></pre>
  
  <p>Any hints on what is going wrong would be appreciated....
  (Targeting .Net 4.5, GalaSoft WPF4 libs v4.0.30319)</p>