Description

A TypeError is thrown by plg_system_debug on every redirect-based action (such as administrator login and logout) in Joomla 6.1.0.

Exception

TypeError: Joomla\Plugin\System\Debug\DataCollector\QueryCollector::__construct():
Argument #2 ($queryMonitor) must be of type Joomla\Database\Monitor\DebugMonitor,
null given, called in .../plugins/system/debug/src/Extension/Debug.php on line 323

Root cause

In plugins/system/debug/src/Extension/Debug.php at line 323, QueryCollector is instantiated without first checking whether $queryMonitor is null. When a redirect fires (e.g. after login/logout), the database monitor has not been initialised, resulting in the type error.

Expected behaviour

The Debug plugin should guard against a null $queryMonitor before instantiating QueryCollector, either skipping the collector silently or initialising the monitor if appropriate.

Suggested fix

phpif ($queryMonitor !== null) {
$this->debugBar->addCollector(
new QueryCollector($this->db, $queryMonitor)
);
}

Steps to reproduce

Enable the System - Debug plugin
Log in or log out of the Joomla administrator backend
Exception is thrown on every such action

Environment

Joomla 6.1.0
PHP 8.4