MythicMob Items -Error when loading and importing.

Summary

I've recently updated from 1.21.4 to 1.21.7 Mythicmob items that use the 'itemstack' format now no longer load, and produce an error on server startup.

[Legacy attachment omitted] (Ones in green layout work. Red layout = do not load)

Steps to reproduce

Start server that has imported mythicmob items.

Current behavior See summary

Intended correct behavior

Mythicmob items should load.

Server log file

Full log: https://mclo.gs/rAODCHJ!

Debug log snippet

Short log: https://mclo.gs/HHiLoUh

Proposed fixes

For /mm import to work again, and for previously imported items to load.