Issue #3262401 by Dom., Berdir: Deprecated function : Return type of...

Issue #3262401 by Dom., Berdir: Deprecated function : Return type of Drupal\linkit\Suggestion\SimpleSuggestion::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize()

Related to #3498223

src/Form/Matcher/AddForm.php

@@ -58,7 +58,7 @@ class AddForm extends FormBase {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, ProfileInterface $linkit_profile = NULL) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?ProfileInterface $linkit_profile = NULL) {
     $this->linkitProfile = $linkit_profile;
 
     $form['#attached']['library'][] = 'linkit/linkit.admin';