TypeError: Drupal\cmrf_form_processor\Plugin\WebformHandler\FormProcessorWebformHandler::getFormProcessor(): Return value must be of type string, null returned

Problem/Motivation

The following error is triggered when adding a new webform handler:

TypeError: Drupal\cmrf_form_processor\Plugin\WebformHandler\FormProcessorWebformHandler::getFormProcessor(): Return value must be of type string, null returned
File "/modules/contrib/cmrf_form_processor/src/Plugin/WebformHandler/FormProcessorWebformHandler.php", line 662, in Drupal\cmrf_form_processor\Plugin\WebformHandler\FormProcessorWebformHandler::getFormProcessor
return $this->configuration['form_processor'];
File "/modules/contrib/cmrf_form_processor/src/Plugin/WebformHandler/FormProcessorWebformHandler.php", line 175, in Drupal\cmrf_form_processor\Plugin\WebformHandler\FormProcessorWebformHandler::buildConfigurationForm
$selected_formprocessor = $this->getFormProcessor();
File "/modules/contrib/webform/src/Form/WebformHandlerFormBase.php", line 205, in Drupal\webform\Form\WebformHandlerFormBase::buildForm
$form['settings'] = $this->webformHandler->buildConfigurationForm($form['settings'], $subform_state);
File "/modules/contrib/webform/src/Form/WebformHandlerAddForm.php", line 35, in Drupal\webform\Form\WebformHandlerAddForm::buildForm
$form = parent::buildForm($form, $form_state, $webform, $webform_handler);
File "/core/lib/Drupal/Core/Form/FormBuilder.php", line 536, in Drupal\Core\Form\FormBuilder::retrieveForm
$form = call_user_func_array($callback, $args);
...
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