code: public function handle(BuildWidgetEvent $event)
      {
          $environment = $event->getEnvironment();
          if (($environment->getDataDefinition()->getName() !== 'tl_metamodel_dcasetting')
              || ($event->getProperty()->getName() !== 'mandatory')
              || (null === $event->getModel()->getId())) {
              return;
          }
  
          $model     = $event->getModel();
          $metaModel = $this->getMetaModelFromModel($model);
          $attribute = $metaModel->getAttributeById($model->getProperty('attr_id'));
          if (null === $attribute) {
              return;
          }
  
          if ($attribute->get('isunique')) {
              Message::addInfo(
                  $this->translator->trans(
                      'tl_metamodel_dcasetting.mandatory_for_unique_attr',
                      [],
                      'contao_tl_metamodel_dcasetting'
                  )
              );
  
              $extra = $event->getProperty()->getExtra();
  
              $extra['disabled'] = true;
  
              $event->getProperty()->setExtra($extra);
  
              $model->setProperty('mandatory', true);
          }
      }
docs: Disable the mandatory checkbox field if the selected attribute is unique.
  
  @param BuildWidgetEvent $event The event.
  
  @return void