Issue #3381893: Site crashes when sending a plain text email that was...

Issue #3381893: Site crashes when sending a plain text email that was formatted by a different module

src/Plugin/Mail/SymfonyMailer.php

@@ -309,9 +309,12 @@ class SymfonyMailer implements MailInterface, ContainerFactoryPluginInterface {
           $email->text($plain, $applicable_charset);
         }
       }
-      else {
+      else if (is_array($message['body'])) {
         $email->text(implode("\r\n", $message['body']), $content_type, $applicable_charset);
       }
+      else {
+        $email->text($message['body'], $content_type, $applicable_charset);
+      }
 
       // Validate that $message['params']['files'] is an array.
       if (empty($message['params']['files']) || !is_array($message['params']['files'])) {