Roundcube - who can help with sessions?

Question

PHP

Hey!
There is Roundecube which works fine while working separately from the project.
But as soon as we connect it to the project, it gives an error

Fatal error: Call to a member function kill() on a non-object in usr/share/roundcubemail/program/include/rcmail.php on line 735

On our site, the user passes authorization, if everything is OK, a page with Roundcube authorization opens where, after entering the login and password from the mail, a white page opens and you give an error that is higher in the logs.
The problem is that we throw data in the session, and in the index.php file (roundecube) we must check if there is data in the session, load the page further, if not, then do a redirect.
So the error crashes if we paste the code in the file
session_start();