Update CpCourseManager.lua

Fixed an issue preventing player from joining in rare case

Author: Flixario

scripts/specializations/CpCourseManager.lua

@@ -367,7 +367,7 @@ function CpCourseManager:onPreDelete()
     if spec then
         g_assignedCoursesManager:unregisterVehicle(self, self.id)
         CpCourseManager.resetCourses(self)
-        spec.courseDisplay:delete()
+        if spec.courseDisplay then spec.courseDisplay:delete() end
         spec.courseDisplay = nil
     end
 end