Steps to reproduce the issue

Just upgraded to PHP 7.2.0

Go to the front end.
Log in.
See php error log.

Actual result

PHP Warning: session_set_save_handler(): Cannot change save handler when session is active in \libraries\joomla\session\storage.php on line 106

PHP Warning: session_set_cookie_params(): Cannot change session cookie parameters when session is active in \libraries\src\Session\Session.php on line 807

System information (as much as possible)

PHP 7.2.0