Issue #3490451 by bluegeek9: RendererInterface::renderPlain is deprecated

Closes #3490451

src/Entity/MessageTemplate.php

@@ -265,7 +265,8 @@ class MessageTemplate extends ConfigEntityBundleBase implements MessageTemplateI
 
       if ($langcode == Language::LANGCODE_NOT_SPECIFIED) {
         // Get the current language code when not specified.
-        $langcode = $language_manager->getCurrentLanguage()->getId();      }
+        $langcode = $language_manager->getCurrentLanguage()->getId();
+      }
 
       if ($this->langcode !== $langcode) {
         $config_translation = $language_manager->getLanguageConfigOverride($langcode, 'message.template.' . $this->id());
@@ -288,7 +289,7 @@ class MessageTemplate extends ConfigEntityBundleBase implements MessageTemplateI
         '#format' => $item['format'] ?? 'plain_text',
         '#langcode' => $langcode,
       ];
-      $text[$key] = \Drupal::service('renderer')->renderPlain($build);
+      $text[$key] = \Drupal::service('renderer')->renderInIsolation($build);
     }
 
     if (isset($delta)) {