WPF Animation: can't bind From property to the ActualWidth - wpf

Question

I'm trying to change a selected TabItem's width in the following way:
<Style TargetType="{x:Type TabItem}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TabItem}">
<Grid x:Name="Root">
<Border x:Name="Border" Background="{TemplateBinding Background}">
<ContentPresenter x:Name="ContentSite" VerticalAlignment="Center" HorizontalAlignment="Stretch" ContentSource="Header" Margin="12,2,12,2"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Trigger.EnterActions>
<BeginStoryboard>
<Storyboard>
<DoubleAnimation Storyboard.TargetProperty="Width"
From="{Binding RelativeSource={RelativeSource AncestorType={x:Type TabItem}, Mode=FindAncestor}, Path=ActualWidth}"
To="200"
Duration="0:0:1"/>
</Storyboard>
</BeginStoryboard>
</Trigger.EnterActions>
</Trigger>
</Style.Triggers>
</Style>
I'm forced to specify the "From" property for the animation because I don't specify the "Width" property for the TabItem explicitly, it is calculated by the custom layout container.
But when I try to bind "From" property of the animation to the "ActualWidth" property of the TabItem, the initialization of 'System.Windows.Controls.TabItem' threws an exception. I tried to move the "Triggers" section inside the "ControlTemplate" section, but with no effect. So I have 2 questions:
1) Why can't the "From" property be bound to the ActualWidth of the TabItem in the way I did it?
2) How can I achieve the desired behaviour?
Any hints would be appreciated.

Answer 1

This cannot be done because WPF framework will freeze the things which would used for the Animation by initially and So if you are trying to Bind on a Freezable, it doest accept that. That's why it was prevented from changing and says,
Cannot freeze this Storyboard timeline tree for use across threads