The pastebin for the log and Registry Event is posted below this text.

 

https://pastebin.com/KEzJvRgG

https://pastebin.com/VUrXR94k

Apparently here:

BlockList.tutorial_slab = new SlabBlock(Block.Properties.from(BlockList.tutorial_slab)).setRegistryName(location("tutorial_slab"))

your BlockList.tutorial_slab block is null when the block registration event is fired..i suggest you use deferred register to manage your registry entries

how do I add a deffered register, sorry I am new to this and im following a tutorial

 

Or is there another option to fix it?

Take a look ate this section of the Docs https://mcforge.readthedocs.io/en/latest/concepts/registries/
In advance, an exemple of DeferredRegister would be something like this:

DeferredRegister<Block> BLOCKS = DeferredRegister.create(ForgeRegistries.ITEMS, MOD_ID);

RegistryObject<Block> COPPER_ORE = BLOCKS.register("copper_ore", () -> new OreBlock(...));

And when you need to access the entries (blocks) you would get them throught the BLOCKS DeferredRegister or if you need only one specific entry you would use COPPER_ORE.get() ... in your case: