Steps to reproduce the issue

Update from 4.4.0 to 5.0.0 with many plugins and based on T4 blank template.
Patch below has fixed the issue on 3 server instances.

Expected result

Website functional.

Actual result

Website failure

Joomla\CMS\Event\Module\ModuleListEvent::onSetModules(): Argument #1 ($value) must be of type array, null given, called in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/AbstractEvent.php on line 225

System information (as much as possible)

Linux based shared web-service

Backtrace:

TypeError in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/Module/ModuleListEvent.php (line 73)

ModuleListEvent->onSetModules(null) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/AbstractEvent.php (line 225)

AbstractEvent->setArgument('modules', null) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/AbstractEvent.php (line 115)

AbstractEvent->__construct('onPrepareModuleList', array('modules' => null, 'subject' => null)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/AbstractImmutableEvent.php (line 51)

AbstractImmutableEvent->__construct('onPrepareModuleList', array('modules' => null, 'subject' => null)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/Module/ModuleEvent.php (line 55)

ModuleEvent->__construct('onPrepareModuleList', array('modules' => null, 'subject' => null)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Event/Module/ModuleListEvent.php (line 48)

ModuleListEvent->__construct('onPrepareModuleList', array(null)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Application/EventAware.php (line 106)

WebApplication->triggerEvent('onPrepareModuleList', array(null)) in /hp/cn/aa/rl/www/kgs-dhs/administrator/cache/t4core/ModuleHelper.php (line 369)

_JModuleHelper::load() in /hp/cn/aa/rl/www/kgs-dhs/administrator/cache/t4core/ModuleHelper.php (line 88)
_JModuleHelper::getModules('debug') in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/Renderer/Html/ModulesRenderer.php (line 50)

ModulesRenderer->render('debug', array('style' => 'none'), null) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/HtmlDocument.php (line 575)

HtmlDocument->getBuffer('modules', 'debug', array('style' => 'none')) in /hp/cn/aa/rl/www/kgs-dhs/templates/t4_bs5_blank/error.php (line 95)

require('/hp/cn/aa/rl/www/kgs-dhs/templates/t4_bs5_blank/error.php') in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/HtmlDocument.php (line 731)

HtmlDocument->_loadTemplate('/hp/cn/aa/rl/www/kgs-dhs/templates/t4_bs5_blank', 'error.php') in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/HtmlDocument.php (line 785)

HtmlDocument->_fetchTemplate(array('template' => 't4_bs5_blank', 'directory' => '/hp/cn/aa/rl/www/kgs-dhs/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php')) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/HtmlDocument.php (line 622)

HtmlDocument->parse(array('template' => 't4_bs5_blank', 'directory' => '/hp/cn/aa/rl/www/kgs-dhs/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php')) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/HtmlDocument.php (line 640)

HtmlDocument->render(false, array('template' => 't4_bs5_blank', 'directory' => '/hp/cn/aa/rl/www/kgs-dhs/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php')) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Document/ErrorDocument.php (line 139)

ErrorDocument->render(false, array('template' => 't4_bs5_blank', 'directory' => '/hp/cn/aa/rl/www/kgs-dhs/templates', 'debug' => false, 'csp_nonce' => null, 'templateInherits' => '', 'params' => object(Registry), 'file' => 'error.php')) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Error/Renderer/HtmlRenderer.php (line 70)

HtmlRenderer->render(object(TypeError)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Exception/ExceptionHandler.php (line 136)

ExceptionHandler::render(object(TypeError)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Exception/ExceptionHandler.php (line 73)

ExceptionHandler::handleException(object(TypeError)) in /hp/cn/aa/rl/www/kgs-dhs/libraries/src/Application/CMSApplication.php (line 336)

CMSApplication->execute() in /hp/cn/aa/rl/www/kgs-dhs/includes/app.php (line 58)
require_once('/hp/cn/aa/rl/www/kgs-dhs/includes/app.php') in /hp/cn/aa/rl/www/kgs-dhs/index.php (line 32) 

Additional comments

Patch:

--- AbstractEvent.php.orig  2023-10-21 17:41:06.314834698 +0200
+++ AbstractEvent.php   2023-10-21 17:36:54.281547361 +0200
@@ -221,7 +221,7 @@
         $methodName1 = 'onSet' . $ucfirst;
         $methodName2 = 'set' . $ucfirst;

-        if (method_exists($this, $methodName1)) {
+        if (method_exists($this, $methodName1) && ($value != null)) {
             $value = $this->{$methodName1}($value);
         } elseif (method_exists($this, $methodName2)) {
             @trigger_error(