Error: "Exception - Call to a member function get_formatted_name() on null" in Moodle Lambda theme

I am using Lambda theme for my Moodle site (version 4.4.1). When a user is trying to enroll in a course, getting error “Exception - Call to a member function get_formatted_name() on null”. Switched on the debug mode and below details appear:
Debug info:
Error code: generalexceptionmessage

Stack trace:

  • line 100 of /theme/lambda2/classes/output/core/course_renderer.php: Error thrown
  • line 84 of /theme/lambda2/classes/output/core/course_renderer.php: call to theme_lambda2\output\core\course_renderer->coursecat_coursebox_content()
  • line 91 of /course/renderer.php: call to theme_lambda2\output\core\course_renderer->coursecat_coursebox()
  • line 104 of /enrol/index.php: call to core_course_renderer->course_info_box().

Appreciate any help to fix this error.

https://help.market.envato.com/hc/en-us/articles/203039054-How-to-contact-an-author