1. What version of Minecraft, Forge and Mod are you running?
   1. Minecraft Version: 1.12.2
   2. Forge Version: forge-1.12.2-14.23.3.2673-installer-win
   3. Mod Version: prefab-1.3.0.20
2. What type of issue is this? Choose one of the following:
   1. Bug (Something doesn't work the way you expect or the game crashes/has an issue)
3. If this is a bug, please provide as much detail as possible for the issue that you are running into. If possible post a log file to PasteBin or another similar site.

crash-2018-05-05_12.16.53-server.txt

When the mineshaft dug the tunnel down, got this crash. Looks like a conflict with some block from either:
SteamAgeRevolution-1.12.2-0.2.0
https://minecraft.curseforge.com/projects/steamagerevolution

or it's core mod:
base-1.12-3.4.2
https://minecraft.curseforge.com/projects/base

Further examination shows it to be Sulphur Ore, which when picked with a diamond pickaxe drops nothing at all. It ought to drop Sulphur dust. I will report the mining issue to that mod's creator(s) as well.

Oops, didn't realise I didn't have the latest version of steam age revolution:
SteamAgeRevolution-1.12.2-0.2.2
base-1.12.2-3.7.1

Same problem persists though.

This is an issue with the core mod of (B.A.S.E). They are not checking for null in their ItemStackUtils class.
As seen here: https://github.com/The-Acronym-Coders/BASE/blob/0244cd5c37a47dab9efdd6778be2f9e4c80fc165/src/main/java/com/teamacronymcoders/base/util/ItemStackUtils.java#L29

This is because they are passing in a null ItemStack from here: https://github.com/The-Acronym-Coders/BASE/blob/0244cd5c37a47dab9efdd6778be2f9e4c80fc165/src/main/java/com/teamacronymcoders/base/materialsystem/blocks/SubBlockOrePart.java#L70

Since there is a possibility that the item stack could be null, they need to be checking for null inside that ItemStackUtils class or creating an empty itemstack instead inside that SubBlockOrePart class.

It looks like they already have a logging call for a situation where the itemstack is null but they aren't it's not being hit properly.

Huh. Weird. I'll pass this along.

I am closing this issue as it is being tracked by the B.A.S.E mod team.