fatal error

I use Paid Memberships Pro. Often, when I change a user’s level (resulting in the addition or subtraction of a course), the site goes into fatal error. Here is the message. Do you know what that means?
Thank you!

Versione di WordPress 5.8.1
Tema corrente: Smart Theme v3 (versione 1.0.15)
Plugin corrente: LearnPress (versione 4.1.3.1)
Versione PHP 7.4.23

Dettagli dell’errore
====================
Un errore di E_ERROR è stato causato nella linea 591 del file /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/user-item/class-lp-user-item-course.php. Messaggio di errore: Uncaught TypeError: Return value of LP_User_Item_Course::get_grade() must be of the type string, null returned in /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/user-item/class-lp-user-item-course.php:591
Stack trace:
#0 /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/user-item/class-lp-user-item-course.php(598): LP_User_Item_Course->get_grade()
#1 /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/admin/views/user/course-progress.php(50): LP_User_Item_Course->is_passed()
#2 /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/admin/lp-admin-functions.php(193): include(‘/mnt/storage/mi…’)
#3 /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/admin/views/user/courses.php(110): learn_press_admin_view()
#4 /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/admin/lp-admin-functions.php(193): include(‘/mnt/storage/mi…’)
#5 /mnt/storage/mindon.it/public_html/wp-content/plugins/learnpress/inc/user/lp-

The page I need help with: [log in to see the link]