Trying to access array offset on value of type null about less.php HOT 1 OPEN

Which version of Less.php are you using? And which version of PHP?

I believe you're on a fairly new version of PHP. The PHP Notice: Trying to access array offset on value of type null message was introduced in PHP 7.4.0. And became a Warning in PHP 8.0.0. To my knowledge this is not yet an "error". This means the program will continue to succeed and output the same CSS as it otherwise would. If you encounter these on user-facing web pages, make sure debugging is off by setting display_errors = Off in PHP itself.

Are you using an old version of Less.php? Less_Parser->NewObj3() was removed in Less.php v3.2.0. I suggest upgrading to Less.php v4.4.0 first.

The $op[0] line of code was also removed in https://gerrit.wikimedia.org/r/c/mediawiki/libs/less.php/+/1013411 and released in v4.3.0. However, to my knowledge this did not fix a bug. I suspect your input.less file might be containing invalid LESS syntax, and this internal warning might be a red herring. Once you upgrade, the warning will be solved, and you will either find success or a clearer error message pointing to the root cause :)

from less.php.