Issue #3292085: Mail Group Type can be deleted while still in use

src/Entity/Form/MailGroupTypeDeleteForm.php

@@ -32,6 +32,37 @@ class MailGroupTypeDeleteForm extends EntityConfirmFormBase {
     return $this->t('Delete');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $count = $this->entityTypeManager
+      ->getStorage('mailgroup')
+      ->getQuery()
+      ->accessCheck(FALSE)
+      ->condition('type', $this->entity->id())
+      ->count()
+      ->execute();
+
+    if ($count) {
+      $message = $this->formatPlural(
+        $count,
+        'There is 1 group of the type %type. You can not remove this group type until you have removed all of the %type groups.',
+        'There are @count groups of the type %type. You can not remove this group type until you have removed all of the %type groups.',
+        ['%type' => $this->entity->label()]
+      );
+
+      $form['#title'] = $this->getQuestion();
+      $form['description'] = [
+        '#markup' => '<p>' . $message . '</p>',
+      ];
+
+      return $form;
+    }
+
+    return parent::buildForm($form, $form_state);
+  }
+
   /**
    * {@inheritdoc}
    */