diff --git a/src/Form/FieldValidationRuleAddForm.php b/src/Form/FieldValidationRuleAddForm.php
index 2626f7b501b722b53f124dda2431126cd1f41ad7..f8318bf1b978cfc8bd704042267117946e363f4a 100644
--- a/src/Form/FieldValidationRuleAddForm.php
+++ b/src/Form/FieldValidationRuleAddForm.php
@@ -50,7 +50,7 @@ class FieldValidationRuleAddForm extends FieldValidationRuleFormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL, $field_name = '') {
+  public function buildForm(array $form, FormStateInterface $form_state, ?FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL, $field_name = '') {
     $form = parent::buildForm($form, $form_state, $field_validation_rule_set, $field_validation_rule);
 
     $form['#title'] = $this->t('Add %label field validation rule', ['%label' => $this->fieldValidationRule->label()]);
diff --git a/src/Form/FieldValidationRuleDeleteForm.php b/src/Form/FieldValidationRuleDeleteForm.php
index d5b67a499b62b255d0130e87e3ee7bf133c6bfbb..06ceedde5a0ac9684da7a4e56d5fe5e95075bc90 100644
--- a/src/Form/FieldValidationRuleDeleteForm.php
+++ b/src/Form/FieldValidationRuleDeleteForm.php
@@ -14,7 +14,7 @@ class FieldValidationRuleDeleteForm extends ConfirmFormBase {
   /**
    * The fieldValidationRuleSet containing the fieldValidationRule to be deleted.
    *
-   * @var \Drupal\field_validation\FieldValidationRuleSetInterface
+   * @var \Drupal\field_validation\FieldValidationRuleSetInterface|null
    */
   protected $fieldValidationRuleSet;
 
@@ -56,7 +56,7 @@ class FieldValidationRuleDeleteForm extends ConfirmFormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL) {
     $this->fieldValidationRuleSet = $field_validation_rule_set;
     $this->fieldValidationRule = $this->fieldValidationRuleSet->getFieldValidationRule($field_validation_rule);
 
diff --git a/src/Form/FieldValidationRuleEditForm.php b/src/Form/FieldValidationRuleEditForm.php
index 3e50352dd88600f369f1daafe5842cf09ce1cdcb..8999b0ca3ed726178e7dec3e785da532e45d0088 100644
--- a/src/Form/FieldValidationRuleEditForm.php
+++ b/src/Form/FieldValidationRuleEditForm.php
@@ -13,7 +13,7 @@ class FieldValidationRuleEditForm extends FieldValidationRuleFormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL, $field_name = '') {
+  public function buildForm(array $form, FormStateInterface $form_state, ?FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL, $field_name = '') {
     $form = parent::buildForm($form, $form_state, $field_validation_rule_set, $field_validation_rule);
 
     $form['#title'] = $this->t('Edit %label rule', ['%label' => $this->fieldValidationRule->label()]);
diff --git a/src/Form/FieldValidationRuleFormBase.php b/src/Form/FieldValidationRuleFormBase.php
index dd5e64d72506189b5837b1120910bb151cc69df5..9e7fed23fe24ccfdc7cda55740bb576be294ef2e 100644
--- a/src/Form/FieldValidationRuleFormBase.php
+++ b/src/Form/FieldValidationRuleFormBase.php
@@ -26,7 +26,7 @@ abstract class FieldValidationRuleFormBase extends FormBase {
   /**
    * The fieldValidationRuleSet.
    *
-   * @var \Drupal\field_validation\FieldValidationRuleSetInterface
+   * @var \Drupal\field_validation\FieldValidationRuleSetInterface|null
    */
   protected $fieldValidationRuleSet;
 
@@ -99,7 +99,7 @@ abstract class FieldValidationRuleFormBase extends FormBase {
    *   An associative array containing the structure of the form.
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The current state of the form.
-   * @param \Drupal\field_validation\FieldValidationRuleSetInterface $field_validation_rule_set
+   * @param \Drupal\field_validation\FieldValidationRuleSetInterface|null $field_validation_rule_set
    *   The field_validation_rule_set.
    * @param string $field_validation_rule
    *   The field_validation_rule ID.
@@ -111,7 +111,7 @@ abstract class FieldValidationRuleFormBase extends FormBase {
    *
    * @throws \Symfony\Component\HttpKernel\Exception\NotFoundHttpException
    */
-  public function buildForm(array $form, FormStateInterface $form_state, FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?FieldValidationRuleSetInterface $field_validation_rule_set = NULL, $field_validation_rule = NULL) {
     $this->fieldValidationRuleSet = $field_validation_rule_set;
     try {
       $this->fieldValidationRule = $this->prepareFieldValidationRule($field_validation_rule);