Steps to reproduce the issue

Go PHP 8.1, Enable debug, and see in backend the warnings

Expected result

No warnings

Actual result

Deprecated: substr(): Passing null to parameter #1 ($string) of type string is deprecated in libraries/vendor/maximebf/debugbar/src/DebugBar/JavascriptRenderer.php on line 792

Deprecated: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in libraries/vendor/maximebf/debugbar/src/DebugBar/JavascriptRenderer.php on line 792

Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in libraries/vendor/maximebf/debugbar/src/DebugBar/JavascriptRenderer.php on line 808

Deprecated: Implicit conversion from float -INF to int loses precision in libraries/vendor/maximebf/debugbar/src/DebugBar/DataFormatter/DataFormatter.php on line 81

System information (as much as possible)

4.0.6, PHP 8.1.1

Additional comments

This file is not in the joomla/joomla-cms repository, where is it so I can propose a MR ?

The first 2 warnings are already fixed upstream here:
https://github.com/maximebf/php-debugbar/blob/master/src/DebugBar/JavascriptRenderer.php#L792-L794
and here:
https://github.com/maximebf/php-debugbar/blob/master/src/DebugBar/JavascriptRenderer.php#L810

For the last one, an (int) needs to be added like this, as arrays can have only int and string indices and floor() returns an int encoded as a float so the indice is an int, which corresponds to the natural array above:

return $sign . round(pow(1024, $base - floor($base)), $precision) . $suffixes[(int) floor($base)];

I have submitted a corresponding PR upstream here:
maximebf/php-debugbar#498