[Administration] Communiquer : correctif MailForm

backend/models/MailForm.php

         if(count($body['Messages']) > 0) {
             $response = $mj->post(\Mailjet\Resources::$Email, ['body' => $body]);
         }
-        
-        $success = $response->success() ;
+
+        $success = (isset($response) && $response) ? $response->success() : false ;
         
         if(!$success) {
             Yii::error($response->getBody(), 'Mailjet');