diff --git a/packages/tree-extension/src/index.ts b/packages/tree-extension/src/index.ts
index 9fd6db057a..7e45145e2e 100644
--- a/packages/tree-extension/src/index.ts
+++ b/packages/tree-extension/src/index.ts
@@ -380,7 +380,7 @@ const notebookTreeWidget: JupyterFrontEndPlugin<INotebookTree> = {
       (editorTracker) => {
         if (editorTracker) {
           editorTracker.widgetAdded.connect((_, editor) => {
-            nbTreeWidget.adWidget(editor);
+            nbTreeWidget.addWidget(editor);
             nbTreeWidget.tabBar.addTab(editor.title);
             nbTreeWidget.currentWidget = editor;
           });