WPF

<Window x:Class="PL.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:mdi="clr-namespace:WPF.MDI;assembly=WPF.MDI"
        Title="MainWindow" Height="400" Width="800">
    <DockPanel>
        <mdi:MdiContainer>
        </mdi:MdiContainer>
    </DockPanel>
</Window>

<mdi:MdiContainer>
</mdi:MdiContainer>

System.NullReferenceException<br />
Object reference not set to an instance of an object.<br />
   at WPF.MDI.MdiContainer.MdiContainer_Loaded(Object sender, RoutedEventArgs e)<br />
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)<br />
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)<br />
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)<br />
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)<br />
   at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent)<br />
   at System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root)<br />
   at MS.Internal.LoadedOrUnloadedOperation.DoWork()<br />
   at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()<br />
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()<br />
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)<br />
   at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)<br />
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)<br />
   at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)