use order user for remark instead of current session user

src/Application/Controller/Admin/AdminOrder.php

@@ -73,7 +73,7 @@ class AdminOrder extends AdminController
     {
         $messageBody = Registry::getRequest()->getRequestEscapedParameter('messagebody');
 
-        if (strlen($messageBody) <= 1) {
+        if (false === is_string($messageBody) || strlen($messageBody) <= 1) {
             Registry::getUtilsView()->addErrorToDisplay(
                 Registry::getLang()->translateString('D3LM_EXC_MESSAGE_NO_LENGTH')
             );
@@ -115,7 +115,7 @@ class AdminOrder extends AdminController
         $remark = oxNew(Remark::class);
         $remark->assign([
             'oxtype'     => AdminUser::REMARK_IDENT,
-            'oxparentid' => $this->order->getUser()->getId(),
+            'oxparentid' => $this->order->getOrderUser()->getId(),
             'oxtext'     => $recipients.PHP_EOL.$messageBody
         ]);
         $remark->save();