Describe the bug
Game had been successfully started, and been played for a while.
But it crashed suddenly.

Crash report
https://pastebin.com/UmcfnP8r

Please complete the following information:

• Minecraft Version: 1.12.2
• Mod Version: 6.3.1

This can be fixed by using a concurrent hash map and simply taking out all the explicit synchronization. It's part of the PR I've created: #608