I have tried two tutorials: here and here. The problem is that I can't get them to work for 1.12 as some methods changed their signatures and a lot of code changes as consequence. On the first link there is a for loop in the class that extends Container that causes a null pointer exception and crashes the game when addSlotToContainer is called. Without the for loop that adds the slots to the container, it works more or less, but I have many other troubles. For example, when the GUI opens, which is the normal crafting table texture, it draws over it the silhouettes of the player armour slots and the shield silhouttes. I don't understand why it is doing it. It is like it overlays it.

 

So instead of asking for anyone to see my code which is as huge as the one in those tutorials to see if they can spot the errors, it's better if I ask if there are any recent tutorials on the block gui concept. I am practicing with tutorials to understand the concepts because I eventually would like to add a custom crafting table to my mod that has custom recipes and also keep the inventory when the player exits the gui. But I can't get those tutorials to work.

I find your lack of Google-fu disturbing: https://wiki.mcjty.eu/modding/index.php/GUI-1.12

Thank you! That one works.