To reproduce: 
  void radTreeView1_EditorInitialized(object sender, TreeNodeEditorInitializedEventArgs e)
        {
            TreeViewTextBoxEditor editor = e.Editor as TreeViewTextBoxEditor;
            BaseTextBoxEditorElement element = (BaseTextBoxEditorElement)editor.EditorElement;
            element.TextChanging+=new TextChangingEventHandler(element_TextChanging);
        }

        void  element_TextChanging(object sender, TextChangingEventArgs e)
        {
            e.Cancel = true;
        }