BuildCraft version: 7.99.20 to 7.99.21
Forge version: 1.12.2-14.23.5.2768
Link to crash report or log: https://pastebin.com/3SZT76gR
multiplayer: multiplayer
Steps to reproduce: Update buildcraft from 7.99.20 to 7.99.21
Additional information: ---

That's odd... it looks like forestry is crashing rather than buildcraft. Can you report this to them instead? ( https://github.com/ForestryMC/ForestryMC/issues )

I thought that this error is more related to buildcraft, because it happened after updating your mod. I created a issues in Forrestry.

Yes, but the crash is happening in forestry code so I'm not sure quite what's happening, or why buildcraft could be causing it.

Right, I should have looked at the forestry code sooner - it's caused by buildcraft's API name changing, which caused forestries @Optional code to break here.