Fatal error: PHP error in file /srv/www/vhosts/no-kill.de/httpdocs/lib/system/event/listener/RGalleryUserPageListener.class.php (20): Non-static method RGallerySystem::getUserItemsListing() should not be called statically, assuming $this from incompatible context

Information:

error message: PHP error in file /srv/www/vhosts/no-kill.de/httpdocs/lib/system/event/listener/RGalleryUserPageListener.class.php (20): Non-static method RGallerySystem::getUserItemsListing() should not be called statically, assuming $this from incompatible context
error code: 0
file: /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/system/WCF.class.php (255)
php version: 5.6.38
wcf version: 1.0.11 pl 1 (Horizon)
date: Thu, 18 Oct 2018 04:06:10 +0000
request: /index.php?page=User&userID=28&s=223a501fd556a5d0942dffc6c585e8926e9293f8
referer:

Stacktrace:

#0 /srv/www/vhosts/no-kill.de/httpdocs/lib/system/event/listener/RGalleryUserPageListener.class.php(20): WCF::handleError(8192, 'Non-static meth...', '/srv/www/vhosts...', 20, Array)
#1 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/system/event/EventHandler.class.php(193): RGalleryUserPageListener->execute(Object(UserPage), 'UserPage', 'assignVariables')
#2 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/page/AbstractPage.class.php(63): EventHandler::fireAction(Object(UserPage), 'assignVariables')
#3 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/page/UserPage.class.php(105): AbstractPage->assignVariables()
#4 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/page/AbstractPage.class.php(77): UserPage->assignVariables()
#5 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/page/UserPage.class.php(170): AbstractPage->show()
#6 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/page/AbstractPage.class.php(36): UserPage->show()
#7 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/page/UserPage.class.php(37): AbstractPage->__construct()
#8 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/util/RequestHandler.class.php(63): UserPage->__construct()
#9 /srv/www/vhosts/no-kill.de/httpdocs/wcf/lib/util/RequestHandler.class.php(91): RequestHandler->__construct('User', Array, 'page')
#10 /srv/www/vhosts/no-kill.de/httpdocs/index.php(8): RequestHandler::handle(Array)
#11 {main}