Sitmap xml creation task error

Hello,
First, thank you for providing and maintaining a sitemap extension. I'm excited to use it.
I've just installed extension version 5.3.1 onto joomla 5.3.4, have used the default sitemap provided by the extension, set it to just map the main menu item, and have received an error message when I attempt to create an xml sitemap through the extension's administrator component interface.

An error has occurred.
0 Attempt to modify property "items" on null

The error message I receive by email is:

A PHP Exception occurred on your site. Here you can find the stack trace:

Exception Type: Error
File: /home/customer/www/mywebsite.com/public_html/components/com_schuweb_sitemap/src/Model/SitemapModel.php
Line: 686
Message: Attempt to modify property "items" on null

#0 /home/customer/www/mywebsite.com/public_html/components/com_schuweb_sitemap/src/Model/SitemapModel.php(202): SchuWeb\Component\Sitemap\Site\Model\SitemapModel->getMenus()
#1 /home/customer/www/mywebsite.com/public_html/components/com_schuweb_sitemap/src/Model/SitemapXmlModel.php(137): SchuWeb\Component\Sitemap\Site\Model\SitemapModel->getNodes()
#2 /home/customer/www/mywebsite.com/public_html/components/com_schuweb_sitemap/src/Model/SitemapXmlModel.php(116): SchuWeb\Component\Sitemap\Site\Model\SitemapXmlModel->createSitemapXml(Object(SchuWeb\Component\Sitemap\Site\Model\SitemapModel))
#3 /home/customer/www/mywebsite.com/public_html/administrator/components/com_schuweb_sitemap/src/Controller/SitemapsController.php(153): SchuWeb\Component\Sitemap\Site\Model\SitemapXmlModel->createxml()
#4 /home/customer/www/mywebsite.com/public_html/administrator/components/com_schuweb_sitemap/src/Controller/SitemapsController.php(129): SchuWeb\Component\Sitemap\Administrator\Controller\SitemapsController->_createxml()
#5 /home/customer/www/mywebsite.com/public_html/libraries/src/MVC/Controller/BaseController.php(730): SchuWeb\Component\Sitemap\Administrator\Controller\SitemapsController->createxml()
#6 /home/customer/www/mywebsite.com/public_html/libraries/src/Dispatcher/ComponentDispatcher.php(143): Joomla\CMS\MVC\Controller\BaseController->execute('createxml')
#7 /home/customer/www/mywebsite.com/public_html/libraries/src/Component/ComponentHelper.php(361): Joomla\CMS\Dispatcher\ComponentDispatcher->dispatch()
#8 /home/customer/www/mywebsite.com/public_html/libraries/src/Application/AdministratorApplication.php(150): Joomla\CMS\Component\ComponentHelper::renderComponent('com_schuweb_sit...')
#9 /home/customer/www/mywebsite.com/public_html/libraries/src/Application/AdministratorApplication.php(205): Joomla\CMS\Application\AdministratorApplication->dispatch()
#10 /home/customer/www/mywebsite.com/public_html/libraries/src/Application/CMSApplication.php(304): Joomla\CMS\Application\AdministratorApplication->doExecute()
#11 /home/customer/www/mywebsite.com/public_html/administrator/includes/app.php(58): Joomla\CMS\Application\CMSApplication->execute()
#12 /home/customer/www/mywebsite.com/public_html/administrator/index.php(32): require_once('/home/customer/...')
#13 {main}
Request information
GET variables
Array
(
    [option] => com_schuweb_sitemap
    [view] => sitemaps
)
POST variables
Array
(
    [filter] => Array
        (
            [search] => 
            [published] => 
            [access] => 
        )

   

Next I made a blank "sitemap.xml" file in the server's public html directory with permissions 644. Trying again to generate the sitemap gave me the same error, and the sitemap.xml file remains empty. Temporarily renaming my htaccess file (generated mostly by akeeba admin tools) to disable it and attempting to generate sitemap again resulted in the same error.

Then I looked at the task, enabled logging, and set a log file for it. I used the task manager to run a test of the task and receive the following message:

Test task (ID: 8)
Task: "Sitemap"
Status: Terminated
No content was returned.

The task log file contains:

#
#<?php die('Forbidden.'); ?>
#Date: 2025-10-05 12:49:02 UTC
#Software: Joomla! 5.3.4 Stable [ Timu ] 30-September-2025 16:00 GMT
#Fields: date   time    priority    message
2025-10-05  12:49:02    INFO    Task> Sitemap: sitemapxml

What am I missing here? Could you please help me figure out how to overcome these errors and correctly generate the xml sitemap?
Thank you,
Ari

It's just a website for a small community of medical professionals, so nothing complex.
The sitemap is only set to map the main menu, so I've attached a screenshot of the main menu, and the manage menus page.
Thanks for any assistance.
Ari