So I've played around a little bit by giving myself vertical slabs with non-existent namespaces.
Well, after I broke one of these the game crashed because the Mod couldn't get the BlockState.

Crash:
The game crashed whilst playing level event
Error: java.lang.NullPointerException: Cannot invoke "net.minecraft.world.level.block.state.BlockState.getSoundType(net.minecraft.world.level.LevelReader, net.minecraft.core.BlockPos, net.minecraft.world.entity.Entity)" because the return value of "cjminecraft.doubleslabs.api.IBlockInfo.getBlockState()" is null