Pretty straightforward... the game crashed when I tried to place stairs that were in a dank/null.

crash-2019-01-24_15.57.14-client.txt

were they vanilla stairs?

They were from Astral Sorcery.

I get the exact same stacktrace (https://paste.dimdev.org/fomeqozocu.mccrash) when placing top-slabs like below. It does not matter if I rightclick adjacent slabs or the cobblestone above. - These are vanilla slabs and vanilla cobblestone.

Same issue in Project Ozone 3, see also #162

So the code I'm working with currently is vastly different from the code in current versions. I'm not sure what it was causing this, but as of testing right now, it appears to be fixed in dev.
github screwed up on the encoding: original gif here: https://puu.sh/DDnpF/11b1fc79a7.gif