Go to line 1165. There might be something very similar to
for i in 0...$data_skills.size
If so, change it to
for i in 1...$data_skills.size
If not, then there should be something like
for SKILL in $data_skills
where SKILL could be skill or just i. If so, below it add
next if SKILL == nil
use skill or i instead of SKILL depending on which as in the one above.