Steps to reproduce the issue

Install J4 nightly.
Install Kunena nightly (www.kunena.org/download)

Login backend
Go to kunena - attachments

Expected result

List

Actual result

An error has occurred.
0 Call to a member function execute() on null

Call stack
--
# | Function | Location
1 | () | JROOT\libraries\vendor\joomla\database\src\DatabaseDriver.php:652
2 | Joomla\Database\DatabaseDriver->execute() | JROOT\libraries\vendor\joomla\database\src\DatabaseDriver.php:1351
3 | Joomla\Database\DatabaseDriver->loadResult() | JROOT\libraries\kunena\attachment\helper.php:707
4 | KunenaAttachmentHelper::getTotalAttachments() | JROOT\administrator\components\com_kunena\template\j4\attachments\default.php:42
5 | include() | JROOT\libraries\src\MVC\View\HtmlView.php:385
6 | Joomla\CMS\MVC\View\HtmlView->loadTemplate() | JROOT\libraries\src\MVC\View\HtmlView.php:191
7 | Joomla\CMS\MVC\View\HtmlView->display() | JROOT\administrator\components\com_kunena\views\attachments\view.html.php:50
8 | KunenaAdminViewAttachments->display() | JROOT\libraries\kunena\view.php:218
9 | KunenaView->displayLayout() | JROOT\libraries\kunena\view.php:136
10 | KunenaView->displayAll() | JROOT\libraries\kunena\controller.php:443
11 | KunenaController->display() | JROOT\libraries\kunena\controller.php:323
12 | KunenaController->executeTask() | JROOT\libraries\kunena\controller.php:214
13 | KunenaController->execute() | JROOT\administrator\components\com_kunena\kunena.php:75
14 | require_once() | JROOT\libraries\src\Dispatcher\LegacyComponentDispatcher.php:69
15 | Joomla\CMS\Dispatcher\LegacyComponentDispatcher::Joomla\CMS\Dispatcher\{closure}() | JROOT\libraries\src\Dispatcher\LegacyComponentDispatcher.php:71
16 | Joomla\CMS\Dispatcher\LegacyComponentDispatcher->dispatch() | JROOT\libraries\src\Component\ComponentHelper.php:380
17 | Joomla\CMS\Component\ComponentHelper::renderComponent() | JROOT\libraries\src\Application\AdministratorApplication.php:116
18 | Joomla\CMS\Application\AdministratorApplication->dispatch() | JROOT\libraries\src\Application\AdministratorApplication.php:159
19 | Joomla\CMS\Application\AdministratorApplication->doExecute() | JROOT\libraries\src\Application\CMSApplication.php:233
20 | Joomla\CMS\Application\CMSApplication->execute() | JROOT\administrator\includes\app.php:63
21 | require_once() | JROOT\administrator\index.php:36

System information (as much as possible)

/**
	 * Load the total count of attachments
	 *
	 * @return boolean
	 * @throws Exception
	 * @since K5.1
	 */
	public static function getTotalAttachments()
	{
		$attachments = null;

		$db = Factory::getDBO();
		$db->getQuery(true)
			->select('*')
			->from($db->quoteName('#__kunena_attachments'));

		try
		{
			$attachments = $db->loadResult();
		}
		catch (RuntimeException $e)
		{
			KunenaError::displayDatabaseError($e);
		}

		return $attachments;
	}

Additional comments