Site Health : An active PHP session was detected

I’m new in WordPress development and I just created my first theme.

In my wp-admin, the "Site Health" tells me that a PHP session has been detected (critical error), here is the message :

A PHP session was created by a session_start() function call. This
interferes with REST API and loopback requests. The session should be
closed by session_write_close() before making any HTTP requests.

I need PHP’s $_SESSION for a theme script, and I added this to my functions.php file for sessions to be properly initialized:

If I delete these lines, the message disappears, but my PHP sessions don’t work anymore.

If I keep these lines, everything seems to be working properly but this message is worrying…

Does anyone have an idea to solve this problem while keeping the ability to use the $_SESSION?

My WP version is 5.5.3 and the PHP version is 7.4.8.

Thank you in advance for your help!