Sometime, I've got this error, somebody know why ? 

thank's for your help

Message: Can't find master page `'.html_document.'` !

File: /home/httpd/vhosts/xxxxxxxxxxxxx.com/httpdocs/ow_core/theme_manager.php

Line: 229

Trace:

#0 /home/httpd/vhosts/xxxxxxxxxxxxx.com/httpdocs/ow_core/html_document.php(580): OW_ThemeManager->getMasterPageTemplate('html_document')

#1 /home/httpd/vhosts/xxxxxxxxxxxxx.com/httpdocs/ow_core/response.php(184): OW_HtmlDocument->render()

#2 /home/httpd/vhosts/xxxxxxxxxxxxx.com/httpdocs/ow_core/application.php(429): OW_Response->respond()

#3 /home/httpd/vhosts/xxxxxxxxxxxxx.com/httpdocs/index.php(83): OW_Application->returnResponse()

#4 {main}

Type: InvalidArgumentException