Fatal Error when the claim is declined.

Steps to reproduce:

1. Go to a node and on `claim` tab, without filling anything, submit it for claim.
2. Go to administrator interface from where you can accept/decline the claim.
3. If you approve the claim, everything works fine. But when you `Decline` the claim, it gives a fatal error.

Here is the error message:

Fatal error: Declaration of Drupal\nodeownership\Form\NodeownershipClaimDeclineForm::submitForm(array &$form, Drupal\nodeownership\Form\FormStateInterface $form_state) must be compatible with Drupal\Core\Form\FormInterface::submitForm(array &$form, Drupal\Core\Form\FormStateInterface $form_state) in [DRUPALROOT]/modules/nodeownership/src/Form/NodeownershipClaimDeclineForm.php on line 13

Thanks!!
~Amit