Error: Call to undefined method Drupal\state_machine\Form\StateTransitionForm::getOperation()

After upgrading Drupal core to 8.6.x from 8.5.x, I'm getting following error:

NOTICE: PHP message: Error: Call to undefined method Drupal\state_machine\Form\StateTransitionForm::getOperation() in .../simple_sitemap/src/Form/FormHelper.php on line 333 #0 .../simple_sitemap/src/Form/FormHelper.php(281): Drupal\simple_sitemap\Form\FormHelper->getFormEntity()"