MediaWiki Issue - PHP Notice: Trying to access array offset on value of type null in .../SectionProfiler.php
Solution 1:
I had the same problem when updating XAMPP from PHP 7.2.2 to PHP 7.4.1, while keeping MediaWiki at version 1.30.0. A comparison of the file
includes/profiler/SectionProfiler.php with the one in the current MediaWiki version 1.34.0 showed that this warning was fixed (commit). So I just replaced the file by the new version and the then messages where gone (the change happens to be backward compatible).