Issue #3339657: Always show summary of validation result if exists

src/Form/UpdateFormBase.php

@@ -66,10 +66,13 @@ abstract class UpdateFormBase extends FormBase {
     foreach ($results as $result) {
       $messages = $result->getMessages();
 
-      if (count($messages) > 1) {
+      // If there's a summary, there's guaranteed to be at least one message,
+      // so render the result as a nested list.
+      $summary = $result->getSummary();
+      if ($summary) {
         $build['#items'][] = [
           '#theme' => $build['#theme'],
-          '#prefix' => $result->getSummary(),
+          '#prefix' => $summary,
           '#items' => $messages,
         ];
       }
@@ -77,7+80,7 @@
         $build['#items'][] = reset($messages);
       }
     }
     $message = $renderer->renderRoot($build);
 
     if ($severity === SystemManager::REQUIREMENT_ERROR) {
       $this->messenger()->addError($message);