wpf expander template: No ItemsPresenter present when created in Collapsed state - wpf

Question

Here's in short a template I use for a wpf TreeViewItem:
<Style x:Key="MyExpanderStyle" TargetType="Expander">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Expander}">
<Border CornerRadius="2" BorderBrush="Gray" BorderThickness="1">
<Grid SnapsToDevicePixels="True">
<Grid.RowDefinitions>
<RowDefinition Name="ExpHeaderRow" Height="20"/>
<RowDefinition Name="ExpContentRow" Height="*"/>
</Grid.RowDefinitions>
<HeaderedContentControl Grid.Row="0" Header="{Binding}" HeaderTemplateSelector="{StaticResource MyHeaderTemplateSelector}"/>
<ContentPresenter x:Name="ExpandSite" Grid.Row="1" Visibility="Collapsed" Margin="{TemplateBinding Padding}">
</ContentPresenter>
</Grid>
</Border>
<ControlTemplate.Triggers>
<Trigger Property="IsExpanded" Value="True">
<Setter Property="Visibility" TargetName="ExpandSite" Value="Visible"/>
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
the style is used in another style applied to a TreeViewItem:
<Style x:Key="tviStyle" TargetType="TreeViewItem"}
...
<Expander Style="{StaticResource MyExpanderStyle}" IsExpanded="{Binding Path=IsExpanded}">
<Expander.Content>
<ItemsPresenter/> <!-- is null when not yet expanded !! -->
</Expander.Content>
</Expander>
...
now in code, i use this:
ItemsPresenter itemsPresenter = itemsControl.FindVisualDescendant<ItemsPresenter>();
which gives me the itemsPresenter, when I have set IsExpanded = true in the viewmodel constructor.
It gives me null, when IsExpanded = False at construction.
So wpf creates UIElements at the point the Expanders state goes to "Expanded". That is ok.
But there are cases when Items get created in a collapsed state, and I want to access the data that is bound on the Expanders Content (the itemsPresenter). The viewmodels have been created, all data is present, except the wpf UIElements.
There are clumsy ways like going Expanded = true, in the viewmodel constructor, then set Expanded = false (e.g. in OnLoaded)
Is there a nicer way? Thank you!