The path for widgets has changed

notebook/static/notebook/js/main.js

@@ -192,7 +192,7 @@ require([
     })
     // BEGIN HARDCODED WIDGETS HACK
     .then(function() {
-        if (!utils.is_loaded('widgets/extension')) {
+        if (!utils.is_loaded('jupyter-js-widgets/extension')) {
             // Fallback to the ipywidgets extension
             utils.load_extension('widgets/notebook/js/extension').catch(function () {
                 console.warn('Widgets are not available.  Please install widgetsnbextension or ipywidgets 4.0');