Use the convenience function

src/docmanager/manager.ts

@@ -189,7 +189,7 @@ class DocumentManager implements IDisposable {
    */
   findWidget(path: string, widgetName='default'): Widget {
     if (widgetName === 'default') {
-      widgetName = this._registry.preferredWidgetFactories(ContentsManager.extname(path)).next().name;
+      widgetName = this._registry.defaultWidgetFactory(ContentsManager.extname(path)).name;
     }
     let context = this._contextForPath(path);
     if (context) {
@@ -278,7 +278,7 @@ class DocumentManager implements IDisposable {
   private _modelFactoryFor(path: string, widgetName: string): DocumentRegistry.IModelFactory<DocumentRegistry.IModel> {
     let registry = this._registry;
     if (widgetName === 'default') {
-      widgetName = registry.preferredWidgetFactories(ContentsManager.extname(path)).next().name;
+      widgetName = registry.defaultWidgetFactory(ContentsManager.extname(path)).name;
     }
     let widgetFactory = registry.getWidgetFactory(widgetName);
     return registry.getModelFactory(widgetFactory.modelName);

src/filebrowser/dialogs.ts

@@ -317,7 +317,7 @@ class CreateFromHandler extends Widget {
       type = fType.contentType || 'file';
     }
     if (!widgetName || widgetName === 'default') {
-      this._widgetName = widgetName = registry.preferredWidgetFactories(ext).next().name;
+      this._widgetName = widgetName = registry.defaultWidgetFactory(ext).name;
     }
 
     // Handle the kernel preferences.