Treeview.items.clear() method return null exception (e.newvalue==null) in selecteditemchanged event

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C#

WPF

First of all I am from Iran and I Cant Speak English Very Good sorry for this.

I made something like OpenFileDialog in Win.Form

and work correctly.

after for better User Interface I try to make it in WPF

I am use TreeView and other controls for made it in both platform(Winform,WPF)

in winform i could do this correctly by below code

private void Folder_FileTreeView_NodeMouseDoubleClick(object sender, TreeNodeMouseClickEventArgs e)
{
    Folder_FileTreeView.Nodes.Clear();//this is necessary to clean first page node, after  get new folders
    if(e.Node.Text=="Desktop")//also this code is necessary to compare node
    {
        //Do something
    }
}

see this link image of my program winform->https://i.stack.imgur.com/aISn8.png

also in WPF I can get text of Item by below code:

private void Folder_FileTreeView_SelectedItemChanged(object sender, RoutedPropertyChangedEventArgs<object> e)
{
    if (e.NewValue!=null)
    {
        StackPanel CustomStackPanel = (StackPanel)((TreeViewItem)e.NewValue).Header;
        TextBlock textBlock = (TextBlock)CustomStackPanel.Children[1];
        nodetext = textBlock.Text;//this line return text of item for compare
    }
    Folder_FileTreeView.Items.Clear();
}

see this link image of my program WPF->https://i.stack.imgur.com/4DGwS.png

if I Don't use Folder_FileTreeView.Items.Clear(); above code return folders without clearing first page, if i use Folder_FileTreeView.Items.Clear(); then e.NewValue returns null.

Please Help to me for use together These codes: Folder_FileTreeView.Items.Clear(); (or clear first page) and get text of selecteditem by user without return null

if help to me for compelete this Dll I can send it free to all programmers. this dll support most of language like germany france china hindi bengali indonesian persian japanese korean arabic portugues latin swede english

Thanks A lot

What I have tried:

I try some Way I try some Way