[MythicMobs 5.7.0] Custom ItemsAdder drops

Summary

Describe the bug encountered in detail.

• ItemsAdder ( 3.6.4-release-3 )
• MythicMobs ( 5.7.0-SNAPSHOT-b4fec839 )

Steps to reproduce

• Spawn a custom mob with ItemsAdder drops https://pastebin.com/duiUv9gr
• Kill the mob, custom item won't be dropped (Stack trace: https://pastebin.com/hUxLyYCG)

Current behavior

When killing a custom mob containing a ItemsAdder item in their Drop list, it throws an error and doesn't drop such item.

Intended correct behavior

After killing the mob, the item should be dropped after proccessing it's chance.

Server log file

https://pastebin.com/5RftWjR5

Debug log snippet

https://pastebin.com/ZW7682a1

Proposed fixes

Describe what you think the issue or any potential fixes may be.