Transitioner NullReference

Using 2.3.1 version: Randomly when dragging a new control (grid, listbox, textbox, button, doesn’t matter) onto my form a nullreference exception in the designer occurs. I don’t drag the controls on the transitioner, it’s just enough for it to be on my application at all.

Actually the issue happens randomly but quite often. Happens when dragging controls on designer, happens when exiting from debugging. Happens when controls in designer get changed.

NullReferenceException: Object reference not set to an instance of an object.

 at MaterialDesignThemes.Wpf.Transitions.Transitioner.ActivateFrame(Int32 selectedIndex, Int32 unselectedIndex)
   at MaterialDesignThemes.Wpf.Transitions.Transitioner.<.ctor>b__14_0(Object sender, RoutedEventArgs args)
   at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
   at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
   at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
   at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
   at System.Windows.BroadcastEventHelper.BroadcastEvent(DependencyObject root, RoutedEvent routedEvent)
   at System.Windows.BroadcastEventHelper.BroadcastLoadedEvent(Object root)
   at MS.Internal.LoadedOrUnloadedOperation.DoWork()
   at System.Windows.Media.MediaContext.FireLoadedPendingCallbacks()
   at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
   at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object resizedCompositionTarget)
   at System.Windows.Media.MediaContext.RenderMessageHandler(Object resizedCompositionTarget)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs, Delegate catchHandler)

[olegsavelos]

Yes on latest build, the exception happens here

  private void ActivateFrame(int selectedIndex, int unselectedIndex)
        {
            if (!IsLoaded) return;

            TransitionerSlide oldSlide = null, newSlide = null;
            for (var index = 0; index < Items.Count; index++)
            {
                var slide = GetSlide(Items[index]);
                if (index == selectedIndex)
                {
                    newSlide = slide;
                    slide.SetCurrentValue(TransitionerSlide.StateProperty, TransitionerSlideState.Current);
                }
                else if (index == unselectedIndex)
                {
                    oldSlide = slide;
                    slide.SetCurrentValue(TransitionerSlide.StateProperty, TransitionerSlideState.Previous);
                }
                else
                {
                    slide.SetCurrentValue(TransitionerSlide.StateProperty, TransitionerSlideState.None);
                }
                Panel.SetZIndex(slide, 0);
            }

            if (newSlide != null)
            {
                newSlide.Opacity = 1;
            }

            if (oldSlide != null && newSlide != null)
            {
                var wipe = selectedIndex > unselectedIndex ? oldSlide.ForwardWipe : oldSlide.BackwardWipe;
                if (wipe != null)
                {
                    wipe.Wipe(oldSlide, newSlide, GetTransitionOrigin(newSlide), this);
                }
                else
                {
                    DoStack(newSlide, oldSlide);
                }

                oldSlide.Opacity = 0;
            }
            else if (oldSlide != null || newSlide != null)
            {
                DoStack(oldSlide ?? newSlide);
                if (oldSlide != null)
                {
                    oldSlide.Opacity = 0;
                }
            }

            _nextTransitionOrigin = null;
        }

when calling

   private TransitionerSlide GetSlide(object item)
        {
            if (IsItemItsOwnContainer(item))
                return (TransitionerSlide)item;

            return (TransitionerSlide)ItemContainerGenerator.ContainerFromItem(item);
        }

My first thought was Virtualization but even when i tried an non-virtualized panel the effect was the same.

Here is the xaml

     <md:Transitioner Grid.Row="1"
                                         SelectedItem="{Binding SelectedItem}"
                                         ItemsSource="{Binding ItemsView}">
                            <md:Transitioner.Resources>
                                <Style TargetType="md:TransitionerSlide" BasedOn="{StaticResource {x:Type md:TransitionerSlide}}">
                                    <Setter Property="BackwardWipe">
                                        <Setter.Value>
                                            <md:FadeWipe Duration="0:0:0.5" />
                                        </Setter.Value>
                                    </Setter>
                                    <Setter Property="ForwardWipe">
                                        <Setter.Value>
                                            <md:FadeWipe Duration="0:0:0.5" />
                                        </Setter.Value>
                                    </Setter>
                                </Style>
                            </md:Transitioner.Resources>
                            <md:Transitioner.ItemTemplate>
                                <DataTemplate>
                                    <ContentControl Content="{Binding Module.View}" />
                                </DataTemplate>
                            </md:Transitioner.ItemTemplate>
                        </md:Transitioner>

[Keboo]

Fixed by #1242