crash-2018-04-25_00.03.05-client.txt
Forge version 1.12.2-forge1.12.2-14.23.3.2655
Mods used:
embers 0.230
Serene Seasons 1.12.2-1.1.4-universal
Soot-0.9
ToughAsNails-1.12.2-3.1.0.113-universal

This is some investigation after another user reported an issue on the Discord.
He reported (and i experienced the same) a crash after trying to open the creative tab of TAN (his crash report: https://pastebin.com/A54tAb7q).
I think his conflicting mod is different, i assume something with how some liquids are registered from the mods?
The crash report points toward some code with buckets and the issue goes away after removing soot.
Posting it here because this is not the only mod that causes this issue apparently.

After some testing with the mods of the other user, we can add Immersive Engineering to the list of mods that cause this issue.
Immersive Engineering version: Immersive Engineering 0.12 - 81
crash-2018-04-25_00.55.00-client.txt