Phosphor widget backward compatibility fix on WidgetRenderer (#3933)

Author: martinRenou

packages/base/src/widget.ts

@@ -983,6 +983,11 @@ export class JupyterLuminoPanelWidget extends Panel {
   private _view: DOMWidgetView;
 }
 
+/**
+ * @deprecated Use {@link JupyterLuminoPanelWidget} instead (Since 8.0).
+ */
+export const JupyterPhosphorPanelWidget = JupyterLuminoPanelWidget;
+
 export class DOMWidgetView extends WidgetView {
   /**
    * Public constructor

python/jupyterlab_widgets/src/renderer.ts

@@ -74,7 +74,8 @@ export class WidgetRenderer
 
     let widget: LuminoWidget;
     try {
-      widget = (await manager.create_view(wModel)).luminoWidget;
+      const view = await manager.create_view(wModel);
+      widget = view.luminoWidget || view.pWidget;
     } catch (err) {
       this.node.textContent = 'Error displaying widget';
       this.addClass('jupyter-widgets');