How to apply filters in WPF TreeView using ListCollectionView?

<Window.DataContext>
    <local:FileManagerViewModel/>
</Window.DataContext>
 
    <Grid >
        <Grid.RowDefinitions>
            <RowDefinition Height="30"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <!--Filter applied to the SfTreeView based on the text typed in this TextBox.-->
        <TextBox x:Name="TextBox" Width="475" BorderThickness="1" Padding="2,0,2,0" Text="{Binding FilterText, UpdateSourceTrigger=PropertyChanged}" Margin="0,3,0,3" />
        <syncfusion:SfTreeView
        Grid.Row="1"
        Width="475"
        Height="550"
        x:Name="treeView"
        BorderThickness="2"
        BorderBrush="LightGray"
        AutoExpandMode="AllNodes"
        FocusVisualStyle="{x:Null}"
        IsAnimationEnabled="True"
        ItemsSource="{Binding CollectionView}"
        ChildPropertyName="SubFiles">
            <syncfusion:SfTreeView.ItemTemplate>
                <DataTemplate>
                    <Grid>
                        <TextBlock VerticalAlignment="Center"
                           Text="{Binding FileName}" />
                    </Grid>
                </DataTemplate>
            </syncfusion:SfTreeView.ItemTemplate>
            <!--Using the SfTreeView loaded event hooks the OnFilterChanged method.-->
            <behavior:Interaction.Triggers>
                <behavior:EventTrigger EventName="Loaded">
                    <local:TreeViewFilterTrigger/>
                </behavior:EventTrigger>
            </behavior:Interaction.Triggers>
        </syncfusion:SfTreeView>
    </Grid>

public class FileManagerViewModel : NotificationObject
{
    #region Fields
 
    private ObservableCollection<FileManager> imageNodeInfo;
 
    public CollectionView CollectionView { get; set; }
 
    public object Item { get; set; }
    #endregion
 
    #region Constructor
 
    public FileManagerViewModel()
    {
        GenerateSource();
        CollectionView = new ListCollectionView(ImageNodeInfo);
    }
 
    #endregion
 
    #region Filtering
    internal delegate void FilterChanged();
    internal FilterChanged filterChanged;
 
    private string filterText = string.Empty;
 
    public string FilterText
    {
        get { return filterText; }
        set
        {
            filterText = value;
            if (filterChanged != null)
                filterChanged();
            RaisePropertyChanged("FilterText");
        }
    }
 
    #endregion
 
    #region Properties
 
    public ObservableCollection<FileManager> ImageNodeInfo
    {
        get { return imageNodeInfo; }
        set { this.imageNodeInfo = value; }
    }
 
 
    #endregion
}
/// <summary>
/// Represents a class that used to trigger filter action for SfTeeView.
/// </summary>
public class TreeViewFilterTrigger : TargetedTriggerAction<SfTreeView>
{
    /// <summary>
    /// Method used to hook the filter method when the SfTreeView is Loaded.
    /// </summary>
    /// <param name="parameter"></param>
    protected override void Invoke(object parameter)
    {
        var viewModel = this.Target.DataContext as FileManagerViewModel;
        viewModel.filterChanged += OnFilterChanged;
    }
 
    /// <summary>
    /// Method to filter the data.
    /// </summary>
    private void OnFilterChanged()
    {
        var viewModel = this.Target.DataContext as FileManagerViewModel;
        viewModel.CollectionView.Filter = (e) =>
        {
            FileManager file = e as FileManager;
            if ((file.FileName.ToLower()).Contains(viewModel.FilterText.ToLower()))
                return true;
            if (file.SubFiles != null)
            {
                foreach (var subFile in file.SubFiles)
                {
                    if (subFile.FileName.ToLower().Contains(viewModel.FilterText.ToLower()))
                        return true;
                    if (subFile.SubFiles != null)
                        foreach (var sub in subFile.SubFiles)
                            if (sub.FileName.ToLower().Contains(viewModel.FilterText.ToLower()))
                                return true;
                }
            }
            return false;
        };
        this.Target.ExpandAll();
 
    }
}