diff -u b/core/lib/Drupal/Core/Render/MainContent/ModalRenderer.php b/core/lib/Drupal/Core/Render/MainContent/ModalRenderer.php
--- b/core/lib/Drupal/Core/Render/MainContent/ModalRenderer.php
+++ b/core/lib/Drupal/Core/Render/MainContent/ModalRenderer.php
@@ -33,7 +33,7 @@
     // otherwise get it from the routing information.
     $options = $request->request->get('dialogOptions', []);
 
-    $modal_selector = $main_content['#modal_selector'] ?: NULL;
+    $modal_selector = $main_content['#modal_selector'] ?? NULL;
     $response->addCommand(new OpenModalDialogCommand($title, $content, $options, NULL, $modal_selector));
     return $response;
   }