Error When Activating Lightstart

I have been getting an error message every time I activate the lightstart plugin.
This has been going on for some time but I have ignored it as the error does not crash the site and disappears from the screen upon refreshing WP Admin.
However, having enabled debugging for another issue I thought it was about time to try and resolve it!

When activating Lightstart, the following error is displayed:
Fatal error: Uncaught Error: Call to a member function get_page_permastruct() on null in /home/customer/www/syndishows.com/public_html/wp-includes/link-template.php:435 Stack trace: #0 /home/customer/www/syndishows.com/public_html/wp-includes/link-template.php(397): _get_page_link(Object(WP_Post), false, false) #1 /home/customer/www/syndishows.com/public_html/wp-includes/link-template.php(197): get_page_link(Object(WP_Post), false, false) #2 /home/customer/www/syndishows.com/public_html/wp-content/plugins/google-sitemap-generator/sitemap.php(538): get_permalink(Object(WP_Post)) #3 /home/customer/www/syndishows.com/public_html/wp-includes/class-wp-hook.php(324): indexnow_after_post_save(‘publish’, ‘private’, Object(WP_Post)) #4 /home/customer/www/syndishows.com/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array) #5 /home/customer/www/syndishows.com/public_html/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #6 /home/customer/www/syndishows.com/public_html/wp-includes/post.php(5521): do_action(‘transition_post…’, ‘publish’, ‘private’, Object(WP_Post)) #7 /home/customer/www/syndishows.com/public_html/wp-includes/post.php(5053): wp_transition_post_status(‘publish’, ‘private’, Object(WP_Post)) #8 /home/customer/www/syndishows.com/public_html/wp-content/plugins/wp-maintenance-mode/includes/classes/wp-maintenance-mode.php(105): wp_publish_post(Object(WP_Post)) #9 /home/customer/www/syndishows.com/public_html/wp-content/plugins/wp-maintenance-mode/includes/classes/wp-maintenance-mode.php(171): WP_Maintenance_Mode->__construct() #10 /home/customer/www/syndishows.com/public_html/wp-includes/class-wp-hook.php(324): WP_Maintenance_Mode::get_instance(”) #11 /home/customer/www/syndishows.com/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters(NULL, Array) #12 /home/customer/www/syndishows.com/public_html/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #13 /home/customer/www/syndishows.com/public_html/wp-settings.php(555): do_action(‘plugins_loaded’) #14 /home/customer/www/syndishows.com/public_html/wp-config.php(107): require_once(‘/home/customer/…’) #15 /home/customer/www/syndishows.com/public_html/wp-load.php(50): require_once(‘/home/customer/…’) #16 /home/customer/www/syndishows.com/public_html/wp-admin/admin.php(34): require_once(‘/home/customer/…’) #17 {main} thrown in /home/customer/www/syndishows.com/public_html/wp-includes/link-template.php on line 435

For users accessing the site with Lightstart enabled, they see the Maintenance Mode page as intended but with the following error at the top:
Notice: Function wp_enqueue_style was called incorrectly. Scripts and styles should not be registered or enqueued until the wp_enqueue_scripts, admin_enqueue_scripts, or login_enqueue_scripts hooks. This notice was triggered by the otter-google-fonts-628 handle. Please see Debugging in WordPress for more information. (This message was added in version 3.3.0.) in /home/customer/www/syndishows.com/public_html/wp-includes/functions.php on line 6085

Any suggestions? Or is it a case of just creating a new template for the plugin to use when enabled?