Issue #3373228: Undefined array key "swiftmailer"

src/Plugin/EmailAdjuster/WrapAndConvertEmailAdjuster.php

@@ -78,7 +78,7 @@ class WrapAndConvertEmailAdjuster extends EmailAdjusterBase implements Container
    * {@inheritdoc}
    */
   public function build(EmailInterface $email) {
-    if ($this->configuration['swiftmailer'] && $this->moduleHandler->moduleExists('swiftmailer')) {
+    if (isset($this->configuration['swiftmailer']) && $this->moduleHandler->moduleExists('swiftmailer')) {
       // Add the CSS library to match Swiftmailer.
       $theme = $email->getTheme();
       $email->addLibrary("$theme/swiftmailer");
@@ -168,7 +168,7 @@ class WrapAndConvertEmailAdjuster extends EmailAdjusterBase implements Container
    *   The wrapped body.
    */
   protected function render(EmailInterface $email, string $body, bool $is_html) {
-    if ($this->configuration['swiftmailer'] && $this->moduleHandler->moduleExists('swiftmailer') && ($message = $email->getParam('legacy_message'))) {
+    if (isset($this->configuration['swiftmailer']) && $this->moduleHandler->moduleExists('swiftmailer') && ($message = $email->getParam('legacy_message'))) {
       $message['body'] = Markup::create($body);
       $message['subject'] = $email->getSubject();
       $render = [