Fix preview in browser and template saving on utf8 columns

[MAILPOET-2969]

lib/API/JSON/v1/Newsletters.php

@@ -300,7 +300,7 @@ class Newsletters extends APIEndpoint {
     }
 
     $newsletter->setBody(
-      $this->emoji->encodeForUTF8Column(MP_NEWSLETTERS_TABLE, 'body', json_decode($data['body']))
+      json_decode($this->emoji->encodeForUTF8Column(MP_NEWSLETTERS_TABLE, 'body', $data['body']), true)
     );
     $this->newslettersRepository->flush();