code: protected function prepareEvent(EventInterface $event)
      {
          foreach (['elementSpec', 'inputSpec'] as $type) {
              if (! $event->getParam($type)) {
                  $event->setParam($type, new ArrayObject());
              }
          }
  
          $elementSpec = $event->getParam('elementSpec');
          $inputSpec   = $event->getParam('inputSpec');
  
          if (! isset($elementSpec['spec'])) {
              $elementSpec['spec'] = [];
          }
          if (! isset($inputSpec['filters'])) {
              $inputSpec['filters'] = [];
          }
          if (! isset($inputSpec['validators'])) {
              $inputSpec['validators'] = [];
          }
      }
docs: Normalizes event setting all expected parameters.
  
  @param EventInterface $event