Error from Learnpress Plugin

hi,
wordpress started sending error message frequently.

please check.

Error Details
=============
An error of type E_ERROR was caused in line 591 of the file /home/admin/web/public_html/wp-content/plugins/learnpress/inc/user-item/class-lp-user-item-course.php. Error message: Uncaught TypeError: Return value of LP_User_Item_Course::get_grade() must be of the type string, null returned in /home/admin/web/public_html/wp-content/plugins/learnpress/inc/user-item/class-lp-user-item-course.php:591
Stack trace:
#0 /home/admin/web/public_html/wp-content/plugins/learnpress/inc/user-item/class-lp-user-item-course.php(598): LP_User_Item_Course->get_grade()
#1 /home/admin/web/public_html/wp-content/plugins/learnpress/inc/admin/views/user/course-progress.php(50): LP_User_Item_Course->is_passed()
#2 /home/admin/web/public_html/wp-content/plugins/learnpress/inc/admin/lp-admin-functions.php(193): include('/home/admin/web...')
#3 /home/admin/web/public_html/wp-content/plugins/learnpress/inc/admin/views/user/courses.php(110): learn_press_admin_view()
#4 /home/admin/web/public_html/wp-content/plugins/learnpress/inc/admin/lp-admin-functions.php(193): include(