diff --git a/src/Form/GridDialog.php b/src/Form/GridDialog.php
index 12cc62f..72fbd81 100644
--- a/src/Form/GridDialog.php
+++ b/src/Form/GridDialog.php
@@ -80,7 +80,7 @@ class GridDialog extends FormBase {
   /**
    * {@inheritDoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state, Editor $editor = NULL) {
+  public function buildForm(array $form, FormStateInterface $form_state, ?Editor $editor = NULL) {
     $form['#attached']['library'][] = 'editor/drupal.editor.dialog';
     $form['#attached']['library'][] = 'ckeditor_bs_grid/dialog';