What happened?

1. Enable Site → Global Configuration → Debug System.
2. In the System - Debug plugin, enable "Query Explains".
3. Trigger any request that causes session_regenerate_id() to run
   (e.g. login/logout), which closes the session DB handler and fires
   a DB disconnect before the request completes.

Version

6.1

Expected result

No error; the query explains the collected or skipped items cleanly.

Actual result

Fatal error:
Call to a member function getLogs() on null
at plugins/system/debug/src/Extension/Debug.php:490
Debug->onAfterDisconnect()
...DatabaseDriver->disconnect()
Session\Handler\DatabaseHandler->close()
session_regenerate_id()

System Information

• Joomla: 6.1.0 (also present on 6.1-dev HEAD)
• PHP: 8.5.2
• DB: MySQL

Additional Comments

Analysis

Debug::__construct() assigns $this->queryMonitor = $this->getDatabase()->getMonitor().
DatabaseDriver::getMonitor() returns null when no monitor is attached
(which is legitimate — monitors are optional). The onAfterDisconnect()
guard only checks JDEBUG, not whether $this->queryMonitor was set, so
line 490 calls getLogs() on null whenever the disconnect event fires
with debug on but no monitor present (notably during session_regenerate_id
via Session\Handler\DatabaseHandler::close()).

Proposed fix

Add a null guard on $this->queryMonitor in the query_explains
branch of onAfterDisconnect():

if ($this->queryMonitor && $this->params->get('query_explains') && \in_array($db->getServerType(), ['mysql', 'postgresql'], true)) {
                                                                                                                                                   
Happy to open a PR against 6.1-dev if desired.