Undefined variable $conf with php 8

During an update testrun ...

PHP Warning:  Undefined variable $conf in /tmp/update_runner.sh.MBtkgvU64J/install/lib/installer_base.lib.php on line 251
PHP Warning:  Trying to access array offset on value of type null in /tmp/update_runner.sh.MBtkgvU64J/install/lib/installer_base.lib.php on line 251