Silverlight ListBox ItemContainerStyle Throws NRE - silverlight

Question

I have the ListBox ItemContainerStyle below. It simply provides for a nice fade-in, and slide out when items are added or removed from the ItemsSource respectively; it works fine.
The problem occurs when I put my list box into a TabControl from the SL toolkit. When I load some items into my listbox, then flip to another tab, then flip back, if I remove an item, I see the animation occur, then right as it's about to finish, I get the following NullReferenceException exception:
at
System.Windows.Controls.ItemContainerGenerator.LayoutStatesManager._layoutStatesGroups_CurrentStateChanged(Object
sender, VisualStateChangedEventArgs e)
at
MS.Internal.CoreInvokeHandler.InvokeEventHandler(Int32
typeIndex, Delegate handlerDelegate,
Object sender, Object args) at
MS.Internal.JoltHelper.FireEvent(IntPtr
unmanagedObj, IntPtr unmanagedObjArgs,
Int32 argsTypeIndex, Int32
actualArgsTypeIndex, String eventName)
To make matters worse, this only happens when I use a custom ItemsPanel, ie a horizontal StackPanel instead of the default VirtualizingPanel.
Here's the complete Template:
<Style x:Key="ListBoxStyleAnimatedAddAndRemove" TargetType="ListBoxItem">
<Setter Property="Padding" Value="3"/>
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Top"/>
<Setter Property="Background" Value="Transparent"/>
<Setter Property="BorderThickness" Value="1"/>
<Setter Property="TabNavigation" Value="Local"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ListBoxItem">
<Grid Background="{TemplateBinding Background}" x:Name="rootGridElement">
<VisualStateManager.VisualStateGroups>
<VisualStateGroup x:Name="LayoutStates">
<VisualState x:Name="AfterLoaded">
<Storyboard>
<DoubleAnimation
Duration="00:00:1"
From="0" To="1"
Storyboard.TargetName="rootGridElement"
Storyboard.TargetProperty="Opacity" />
</Storyboard>
</VisualState>
<VisualState x:Name="BeforeUnloaded" />
<VisualStateGroup.Transitions>
<VisualTransition From="AfterLoaded" To="BeforeUnload" GeneratedDuration="0:0:0.5">
<Storyboard>
<DoubleAnimation
Duration="00:00:0.25"
By="235"
Storyboard.TargetName="contentProjection"
Storyboard.TargetProperty="GlobalOffsetY" />
</Storyboard>
</VisualTransition>
</VisualStateGroup.Transitions>
</VisualStateGroup>
<VisualStateGroup x:Name="CommonStates">
<VisualState x:Name="Normal" />
<VisualState x:Name="MouseOver">
<Storyboard>
<DoubleAnimation Duration="0" To=".35" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="fillColor"/>
</Storyboard>
</VisualState>
<VisualState x:Name="Disabled">
<Storyboard>
<DoubleAnimation Duration="0" To=".55" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="contentPresenter"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="SelectionStates">
<VisualState x:Name="Unselected"/>
<VisualState x:Name="Selected">
<Storyboard>
<DoubleAnimation Duration="0" To=".75" Storyboard.TargetProperty="Opacity" Storyboard.TargetName="fillColor2"/>
</Storyboard>
</VisualState>
</VisualStateGroup>
<VisualStateGroup x:Name="FocusStates">
<VisualState x:Name="Focused">
<Storyboard>
<ObjectAnimationUsingKeyFrames Duration="0" Storyboard.TargetProperty="Visibility" Storyboard.TargetName="FocusVisualElement">
<DiscreteObjectKeyFrame KeyTime="0">
<DiscreteObjectKeyFrame.Value>
<Visibility>Visible</Visibility>
</DiscreteObjectKeyFrame.Value>
</DiscreteObjectKeyFrame>
</ObjectAnimationUsingKeyFrames>
</Storyboard>
</VisualState>
<VisualState x:Name="Unfocused"/>
</VisualStateGroup>
</VisualStateManager.VisualStateGroups>
<Rectangle x:Name="fillColor" Fill="#FFBADDE9" IsHitTestVisible="False" Opacity="0" RadiusY="1" RadiusX="1"/>
<Rectangle x:Name="fillColor2" Fill="#FFBADDE9" IsHitTestVisible="False" Opacity="0" RadiusY="1" RadiusX="1"/>
<ContentPresenter x:Name="contentPresenter" Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="{TemplateBinding Padding}">
<ContentPresenter.Projection>
<PlaneProjection x:Name="contentProjection">
</PlaneProjection>
</ContentPresenter.Projection>
</ContentPresenter>
<Rectangle x:Name="FocusVisualElement" RadiusY="1" RadiusX="1" Stroke="#FF6DBDD1" StrokeThickness="1" Visibility="Collapsed"/>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>