NullPointerException during import of staff in a course

Sentry Issue: QUEUE-1R

NullPointerException: None
    at nl.tudelft.ewi.queue.model.User.addRole(User.java:154)
    at nl.tudelft.ewi.queue.service.CourseService.addCourseStaff(CourseService.java:120)
    at nl.tudelft.ewi.queue.controller.CourseController.importParticipants(CourseController.java:283)
...
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