WPF Animation "Cannot freeze this Storyboard timeline tree for use across threads"

I currently have a listbox that has its selected item bound to a property on my ViewModel. Whenever the selected item isn't null I want to perform an animation on it. However I keep getting the following error "Cannot freeze this Storyboard timeline tree for use across threads" and from research sort of understand why this is happening. However I am unsure of what approach I need to take to get the behavior I want.

<Storyboard x:Key="ShowItemEdit">
    <DoubleAnimation
        Storyboard.TargetName="lstItemList"
        Storyboard.TargetProperty="ListBox.Width"
        To="{Binding ActualWidth, ElementName=UserControl}"
        Duration="0:0:0.40" />
    ...
</Storyboard>

<Style x:Key="ListStyle">
    <Style.Triggers>
        <DataTrigger Binding="{Binding SelectedItem, Converter={StaticResource IsNullConverter}}" Value="False">
            <DataTrigger.EnterActions>
            <BeginStoryboard Storyboard="{StaticResource ShowItemEdit}" />
        </DataTrigger.EnterActions>
        </DataTrigger>
     </Style.Triggers>
</Style>

<ListBox x:Name="lstItemList" Style={StaticResource ListStyle}" SelectedItem="{Binding SelectedItem}">
    ...
</ListBox>