TreeView crashes when item/s get removed

[RefactoringRocks]

When we use the Style "MaterialDesignTreeViewItem" and remove items from the tree view (for example via MVVM -> ObservableCollection) the Style crashes the application with a NullReferenceException.

<VisualState x:Name="Expanded">
    <Storyboard>
        <DoubleAnimation Storyboard.TargetProperty="Opacity" Storyboard.TargetName="ItemsHost"
                     To="1" Duration="0"/>
        <!-- TODO: This line here breaks the whole app when we remove items! -->
        <!--<DoubleAnimation Storyboard.TargetProperty="Tag" Storyboard.TargetName="ItemsScrollViewer"
                     To="1" Duration="0"/>-->
        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="ItemsHost" Storyboard.TargetProperty="Visibility">
            <DiscreteObjectKeyFrame KeyTime="0" Value="{x:Static Visibility.Visible}" />
        </ObjectAnimationUsingKeyFrames>
    </Storyboard>
</VisualState>

<ScrollViewer Grid.Column="1" Grid.Row="1"
      x:Name="ItemsScrollViewer"
      Margin="-16 0 0 0"
      VerticalScrollBarVisibility="Hidden"
      Grid.ColumnSpan="2">
    <ScrollViewer.Tag>
        <system:Double>0.0</system:Double>
    </ScrollViewer.Tag>
    <!-- TODO: This line here breaks the whole app when we remove items! -->
    <!--<ScrollViewer.Height>
        <MultiBinding Converter="{StaticResource MathMlpMultipleConverter}">
            <Binding ElementName="ItemsHost" Path="ActualHeight"/>
            <Binding RelativeSource="{RelativeSource Self}" Path="Tag"/>
        </MultiBinding>
    </ScrollViewer.Height>-->
    <ItemsPresenter x:Name="ItemsHost"
            VerticalAlignment="Top"
            Opacity="0"
            Visibility="Collapsed"/>
</ScrollViewer>

The commented out code parts are throwing the exception.

[ButchersBoy]

😢

[l1pton17]

@RefactoringRocks could you please provide an example? I have tried to obtain this error, but i could not.
I have made a pull request #160, you can check Domain\TreesViewModel.cs file in MainDemo.Wpf project.

[ButchersBoy]

@RefactoringRocks are you able to provide an example of this breaking?

[ButchersBoy]

Closing due to inactivity