Steps to reproduce the issue

• add the $root_user value to the configuration.php (see: https://docs.joomla.org/How_do_you_recover_or_reset_your_admin_password%3F#Method_1:_configuration.php_file)
• Login into the backend
• notice the message
• try the link to remove the root user entry automatically (Click here to try to do it automatically.)

http://www.example.org/administrator/index.php?option=com_config&task=application.removeroot&38c457982fff49a4c11a46989cd3b10f=1

Expected result

the $root_user var gets removed from the configuration.php.

Actual result

Warning: Creating default object from empty value in JROOT\administrator\components\com_config\config.php on line 28

Fatal error: Call to undefined method stdClass::execute() in JROOT\administrator\components\com_config\config.php on line 31

Additional comments

It works with 2.5.28