code: public function pickerAction(Request $request): RedirectResponse
      {
          $extras = [];
  
          if ($request->query->has('extras')) {
              $extras = $request->query->get('extras');
  
              if (!\is_array($extras)) {
                  throw new BadRequestHttpException('Invalid picker extras');
              }
          }
  
          $config = new PickerConfig($request->query->get('context'), $extras, $request->query->get('value'));
          $picker = $this->get('contao.picker.builder')->create($config);
  
          if (null === $picker) {
              throw new BadRequestHttpException('Unsupported picker context');
          }
  
          return new RedirectResponse($picker->getCurrentUrl());
      }
docs: Redirects the user to the Contao back end and adds the picker query parameter.
  It will determine the current provider URL based on the value, which is usually
  read dynamically via JavaScript.
  
  @throws BadRequestHttpException
  
  @Route("/contao/picker", name="contao_backend_picker")