potion effects skills will throw errors in console if combined with @target and onTimer

Summary The potion effect skill will spam errors if combined with @target and onTimer

Steps to reproduce

1. Use the skill from the item in this pastebin https://pastebin.com/CTnVfgnr
2. Spawn any mob, this is the one i used: https://pastebin.com/81dv4EKV
3. Use the item's skill on the mob, it won't work

Current behavior Nothing is happening as you can see in this video: https://www.youtube.com/watch?v=3zrlQCSaqwo&feature=youtu.be

The skill is not working and it will throw errors in console

Intended correct behavior The skill should work and the mob should get the slowness effect

Server log file https://pastebin.com/CLD2EMeQ

Debug log snippet https://pastebin.com/FDk5h4LC