I'm getting a fatal error when I go 'Manage categories' on admin
It seems other menu working fine like manage products but manage categories is not work
Fatal error: require_once(): Failed opening required 'Mage/Adminhtml/Block/Catalog/Category/Tabs.php' (include_path='/home/mywebsite/www/includes/src:.:/usr/local/lib/php') in /home/mywebsite/www/includes/src/Magedev_Productposition_Block_Catalog_Category_Tabs.php on line 8
What should I do to fix this error?