code: public function presetVariantBase(PreEditModelEvent $event)
      {
          $model = $event->getModel();
  
          // Check of we have the driver from MetaModels. Only these request are from interest.
          if (!$model instanceof Model) {
              return;
          }
  
          // Get the item and check the context.
          $nativeItem = $model->getItem();
          $metaModel  = $nativeItem->getMetaModel();
  
          if ($metaModel->hasVariants() && (!$nativeItem->get('vargroup'))) {
              $nativeItem->set('varbase', '1');
          }
      }
docs: Check the items before the edit start. If there is a item with variant support and a empty vargroup it must be a
  base. So set the varbase to 1.
  
  @param PreEditModelEvent $event The event with the model.
  
  @return void