So this is a long time issue since courseid changed in kfc-6.0.3.7.
I've been test on 1.50c/1.50d+kfc-6.1.x/kfc-6.2.x/kfc-6.0.x/kfc-6.0.3.7, they got the same error. But if changed to kfc-6.0.3.6 then it work.

I added this to bypass the progress when it can not find proper IDs but will lose the Skill Analyzer data.
if(skillData && 'courses' in skillData) {
My data was created after kfc-6.0.3.7 so it dosen't have problem, but I test some data created before kfc-6.0.3.7 from other people, it will appear this error when you update to latest or some version after kfc-6.0.3.7.
In sometimes it will broke the savedata and fill the file with some label and savedata database will boom to like 20-60mb

So this is a long time issue since courseid changed in kfc-6.0.3.7.
I've been test on 1.50c/1.50d+kfc-6.1.x/kfc-6.2.x/kfc-6.0.x/kfc-6.0.3.7, they got the same error. But if changed to kfc-6.0.3.6 then it work.
I added this to bypass the progress when it can not find proper IDs but will lose the Skill Analyzer data.
My data was created after kfc-6.0.3.7 so it dosen't have problem, but I test some data created before kfc-6.0.3.7 from other people, it will appear this error when you update to latest or some version after kfc-6.0.3.7.
In sometimes it will broke the savedata and fill the file with some label and savedata database will boom to like 20-60mb