Updated to the most recent Fabric Build in my mod pack and started getting a crash on world load.
Something about:
Int2ObjectMap contained two equal IDs 0 (minecraft:air/837 -> minecraft:air/0)!

Tested with:

• Fabric Loader 0.4.8 154 Minecraft 1.14.2
• Fabric API Build 173
• Cotton 0.6.7, as #24 of Cotton is still open and I can't seem to find any those hot fix builds yet ;)

Pastebin Crash (No mods besides dependencies)

The hotfix builds have replaced the broken builds. I've been running into this air issue for a while and I have no idea what's been causing it.

I could not load my saved worlds again and after long investigating I isolated this mod as the cause. I'm very sorry because I tend to use the farming hoppers a lot.
https://pastebin.com/2T4VtqKS

I still don't know what's going on here.

I only found a hint about "air" in the DesertCropFeature.java
Maybe disabling this class in a test build for 1.14.2 leads to loadable world saves?

No, this is happening outside of the air bug too. I think it might be something with Fabric API.

Ok. I see.

I am having the same issue, but with different IDs (minecraft:air/696 -> minecraft:air/0). I'm also running Fabric 0.4.8 for MC 1.14.2. Not sure if that info will help at all.

I believe this may be an issue with the new fabric API.

This is indeed a Fabric API issue.