In 1.14.3 pre-1, mojang introduced async json loading for vanilla data. This broke the loot table api's mixin at 05f11b7#diff-0a1c21c182b8708aff532f69a5f4feffR40

The correct way is to inject at the apply method that takes (T, Executor).

Example error log:
https://hastebin.com/metaquketa.rb
https://hastebin.com/hadetobere.makefile

Reports on discord:
https://discordapp.com/channels/507304429255393322/507982478276034570/587759437687554078 (includes a log snippet)
https://discordapp.com/channels/507304429255393322/507982463449169932/587859116484067328
https://discordapp.com/channels/507304429255393322/507982478276034570/587918350437318656

Another one: https://pastebin.com/AEa4qMV1

Fixed in f0d5680