MI2U crashes on maps with custom items

Error logs when joining a server with custom items: 

<img width="720" height="497" alt="Image" src="https://github.com/user-attachments/assets/b591c3c2-32b1-454b-b98f-32613375c7e4" />

I fixed the crash for myself by simply iterating through each itemCoder item, but I don't think that's a real fix and proper custom content support should be added.