TYPO3 Core

This error is logged:

Core: Exception handler (WEB): Uncaught TYPO3 Exception:
Argument 1 passed to GuzzleHttp\HandlerStack::push() must be callable, null given,
called in /var/www/html/public_html/typo3/sysext/core/Classes/Http/Client/GuzzleClientFactory.php on line 55 | TypeError thrown
in file /var/www/html/typo3app/vendor/guzzlehttp/guzzle/src/HandlerStack.php in line 143.
Requested URL: http://127.0.0.1:51361/typo3/index.php?route=%%2Fajax%%2Fdashboard%%2Fwidget%%2Fcontent&token=--AnonymizedToken--&widget=t3securityAdvisories

I remarked that my usage in the provided extension is not complete and the method handle has to return a callable.

The examples on https://docs.typo3.org/m/typo3/reference-coreapi/master/en-us/ApiOverview/Http/Index.html are working as simple hook but never fulfill the requirements for a guzzle-middleware. Therefore the hook fails in a stack of working middlewares.
Further info: https://docs.guzzlephp.org/en/latest/handlers-and-middleware.html#handlers

While the linked TYPO3-documentation can be regarded as completely misleading and wrong the requirement fore a simple hook does exist and is quite useful.
My combination of the examples of the linked TYPO3-documentation is wrong, nevertheless the requirement for a simple hook does exist and is quite useful, btw. it's used in production already and I've another production candidate for the hook.

As painful as it is for me: the issue can be closed.

Thanks for the answer, I ended up with a simple PSR-15 middleware which is easier and sufficient and also not bound to Guzzle which wouldn't be so logical.
https://docs.typo3.org/m/typo3/reference-coreapi/9.5/en-us/ApiOverview/RequestHandling/Index.html