Hello, I've experienced this issue more than once: https://gist.github.com/Dockter/60fe565b2d874c93c9aed89763e4b210

Details:
Minecraft 1.12.2
Optifine: OptiFine_1.12.2_HD_U_E2
Buildcraft: buildcraft-7.99.19-pre2-3ae9ce378231 (its a compiled version of latest 1.12.x branch)

Reproduce: When we install our modpack, Almura 3, if we enter the game on first launch, and attempt to add our x128 texture pack the above error is generated. If we do a first launch, close the game, then re-launch and try to apply it, it works fine.

I haven't dug into the buildcraft texture method within the chain just thought maybe you folks would know something about this before I dug into it further.

Thanks
-Almura Dev Team-

Can you try disabling "useSwappableSprites" in the bc config? That should fix the crashes.

Although I'm not sure why it doesn't crash all the time...