Whenever I try to open a certain recipe it crashes the gui and even removes the FNEI button on the top left. It's a recipe using Synthesis Gas with the following mod list - https://pastebin.com/7AY3ffFz

Having the same error i found that the bug exists if omnimatter is loaded

confirm. trace:
210.508 Script @FNEI/core/Debug.lua:98: [FNEI][ERROR] <FNCustomEvents> FNEI: event main choose-elem-button item return error!
210.513 Script @FNEI/core/Debug.lua:98: [FNEI][ERROR] <FNCustomEvents> FNEI/scripts/recipe/controller.lua:430: bad argument #1 to 'pairs' (table expected, got nil)

reverting to version 0.1.9 corrected issue in personal playthrough.

omg ty very much :)

(although now, some recipe usages can't be looked-up) can't seem to trace specific mod interaction either as there's over 150 mods...if i find out more, i'll update)

I also tried deleting mod folder and redownloading mods.
Currently unable to reproduce this error. Possibly a corrupted mod-version, or my initial circumstances (such as various configuration_Changed events mean that reproducing this scenario to trace the cause further has been...rather difficult. if i find out any more, i'll return.

It's never fun to delete your mod folder, but instead you can try and relocate it to a safe place, and download mods a-fresh. see if FNEI fails (on a new save ONLY!) don't reload the save where it was crashing or use "sync mods" feature. But instead try and get your setup as close to what it was before without using that in-game mechanic and see if it still fails. (using the sync-mods feature) may contribute to the error re-occuring by copying modsettings, and any other issues that could have contributed to bug.

think I have run into this or a similar bug on a pyblock setup

root cause appears to be that FNEI assumes the names of recipe-category and resource-category will be disjoint. pycoalprocessing has both for "borax" and "niobium". It additionally adds recipies for "raw-borax" and "niobium-ore", which the ores spit out, to serve as upgrades later.

when FNEI walks the two lists, it adds imposter recipes for the minable result, but since it only uses the category name as the lookup, it ends up checking the non-mining recipe against the borax-mine (or niobium-mine), seeing the resource-miner entry added by CraftCategory:create_crafting_category_list, and explodes when it looks for a mining_time on the recipe

fixed in 0.2.2

Second error (FNEI/scripts/recipe/controller.lua:430: bad argument #1 to 'pairs' (table expected, got nil)) fixed in 0.2.3