Fix sender id.

main/inc/lib/message.lib.php

@@ -296,7 +296,7 @@ class MessageManager
         $parent_id = 0,
         $editMessageId = 0,
         $topic_id = 0,
-        $sender_id = null,
+        $sender_id = 0,
         $directMessage = false,
         $forwardId = 0,
         $smsParameters = []
@@ -519,7 +519,7 @@ class MessageManager
         $receiver_user_id,
         $subject,
         $message,
-        $sender_id = null,
+        $sender_id = 0,
         $sendCopyToDrhUsers = false,
         $directMessage = false,
         $smsParameters = [],

main/inc/lib/usermanager.lib.php

@@ -527,7 +527,7 @@ class UserManager
                     if (!empty($notification) && isset($notification['admins']) && is_array($notification['admins'])) {
                         foreach ($notification['admins'] as $adminId) {
                             $emailSubjectToAdmin = get_lang('UserAdded').': '.api_get_person_name($firstName, $lastName);
-                            MessageManager::send_message_simple($adminId, $emailSubjectToAdmin, $emailBody);
+                            MessageManager::send_message_simple($adminId, $emailSubjectToAdmin, $emailBody, $userId);
                         }
                     }
                 }