Help resolving the root cause for Exception: Item id not set

[socrates3992]

Hi - I am writing a mod to add a shotgun item to Minecraft Java Edition. I am using forge 54.1.2 and Minecraft 1.21.4. I am using gradlew and my mod compiles fine but when I launch Minecraft with the mod loaded, it crashes. I do have experience with Java but this is my very first time writing a mod.

The Exception thrown is below:

[11Mar2025 12:15:16.642] [Render thread/ERROR] [net.minecraftforge.fml.javafmlmod.FMLModContainer/]: Exception caught during firing event: Item id not set
    Index: 1
    Listeners:
        0: NORMAL
        1: ASM: net.minecraftforge.registries.DeferredRegister$EventDispatcher@4f9c6f1d handleEvent(Lnet/minecraftforge/registries/RegisterEvent;)V
java.lang.NullPointerException: Item id not set
    at java.base/java.util.Objects.requireNonNull(Objects.java:259)
    at TRANSFORMER/[email protected]/net.minecraft.world.item.Item$Properties.effectiveDescriptionId(Item.java:465)
    at TRANSFORMER/[email protected]/net.minecraft.world.item.Item.<init>(Item.java:111)
    at TRANSFORMER/[email protected]/com.example.shotgunmod.ShotgunItem.<init>(ShotgunItem.java:19)
    at TRANSFORMER/[email protected]/com.example.shotgunmod.ModItems.lambda$static$0(ModItems.java:13)
    at TRANSFORMER/[email protected]/net.minecraftforge.registries.DeferredRegister$EventDispatcher.lambda$handleEvent$0(DeferredRegister.java:381)
    ...
   
My class, ModItems.java has the following function defined on line 13:

13: public static final RegistryObject<Item> SHOTGUN = ITEMS.register("shotgun", () -> new ShotgunItem(new Item.Properties().durability(100)));

Then, my ShotgunItem class has the following constructor which calls the super class:

public class ShotgunItem extends Item {
    public ShotgunItem(Properties properties) {
        super(properties); // line 19
    }

So the crash is coming from super(). Can anyone guide me on the proper way to set the Item ID? Is that the issue here? Thank you for your help.

Blake
 

[socrates3992]

Digging in to the source, in this case net.minecraft.world.item.Item.Properties, I see these functions are the error source:

        protected String effectiveDescriptionId() {
            return this.descriptionId.get(Objects.requireNonNull(this.id, "Item id not set"));
        }

        public ResourceLocation effectiveModel() {
            return this.model.get(Objects.requireNonNull(this.id, "Item id not set"));
        }


Can anyone share more information on how to set the Id properly? Is this something I have to do explicitly by calling the setId() function? My understanding is the ID is set implictly with these calls:

 

    public static final DeferredRegister<Item> ITEMS = DeferredRegister.create(ForgeRegistries.ITEMS, ShotgunMod.MODID);
    private static final Item.Properties p = new Item.Properties().durability(100).stacksTo(1);
    public static final RegistryObject<Item> SHOTGUN = ITEMS.register("shotgun", () -> new ShotgunItem(p));

From net.minecraft.world.item.Item.Properties;

 public Item.Properties setId(ResourceKey<Item> pId) {
            this.id = pId;
            return this;
  }

Thank you for your help

[socrates3992]

I will keep adding to this thread with more information in case anyone can help, or at the very least I can keep my troubleshooting organized. I decided to downgrade to 54.1.0 in the hopes that this would fix the issue but it didn't. At least now I am on a "recommended" version.

The crash report did confirm my earlier post that the Exception is coming from effectiveDescriptionId(). I'll continue to see if I can find a way to set the ID manually.

 

Caused by: java.lang.NullPointerException: Item id not set
        at java.base/java.util.Objects.requireNonNull(Objects.java:259) ~[?:?]
        at TRANSFORMER/[email protected]/net.minecraft.world.item.Item$Properties.effectiveDescriptionId(Item.java:465) ~[forge-1.21.4-54.1.0-client.jar!/:?]
        at TRANSFORMER/[email protected]/net.minecraft.world.item.Item.<init>(Item.java:111) ~[forge-1.21.4-54.1.0-client.jar!/:?]
        at TRANSFORMER/[email protected]/com.example.shotgunmod.ShotgunItem.<init>(ShotgunItem.java:19) ~[shotgunmod-1.0.0.jar!/:1.0.0]
        at TRANSFORMER/[email protected]/com.example.shotgunmod.ModItems.lambda$static$0(ModItems.java:15) ~[shotgunmod-1.0.0.jar!/:1.0.0]
        at TRANSFORMER/[email protected]/net.minecraftforge.registries.DeferredRegister$EventDispatcher.lambda$handleEvent

 

 

 

[socrates3992]

Hi here is an update. I was able to fix the code so my mod does not crash Minecraft. Please understand that I am new to modding but I honestly am having a hard time understanding how anyone can get this to work without having extensive programming and debugging experience as well as searching across the Internet, multiple gen AI bots (claude, grok, openai), and examining source code hidden in the gradle directory and in various github repositories. I guess I am wrong because clearly there are thousands of mods so maybe I am just a newbie. Ok, rant over, here is a step by step summary so others can save the 3 days it took me to figure this out.

 

1. First, I am using forge 54.1.0 and Minecraft 1.21.4
2. I am creating a mod to add a shotgun to Minecraft
3. After creating the mod and compiling it, I installed the .jar file to the proper directory in Minecraft and used 1.21.4-forge-54.1.0
4. The mod immediately crashed with the error: Caused by: java.lang.NullPointerException: Item id not set
5. Using the stack trace, I determined that the Exception was being thrown from the net.minecraft.world.item.Item.Properties class
6. It seems that there are no javadocs for this class, so I used IntelliJ which was able to provide a decompiled version of the class, which I then examined to see the source of the error. Side question: Are there javadocs?
7. This method, specifically, was the culprit:

protected String effectiveDescriptionId() {
     return this.descriptionId.get(Objects.requireNonNull(this.id, "Item id not set"));
 }

8. Now my quest was to determine how to set this.id. Looking at the same source file, I determined there was another method:

 public Item.Properties setId(ResourceKey<Item> pId) {
            this.id = pId;
            return this;
  }

9. So now, I need to figure out how to call setId(). This required working backwards a bit.

Starting from the constructor, I stubbed out the variable p which is of type Item.Properties

public static final RegistryObject<Item> SHOTGUN = ITEMS.register("shotgun", () -> new ShotgunItem(p));

Rather than putting this all on one line, I split it up for readability like this:

private static final Item.Properties p = new Item.Properties().useItemDescriptionPrefix().setId(rk);

Here is was the missing function, setId(), which takes a type of ResourceKey<Item>. My next problem is that due to the apparent lack of documentation (I tried searching the docs on this site) I could not determine the full import path to ResourceKey. I did some random searching on the Internet and stumbled across a Github repository which gave two clues:

import net.minecraft.resources.ResourceKey;
import net.minecraft.resources.ResourceLocation;

Then I created the rk variable like this:

private static ResourceKey<Item> rk = ResourceKey.create(Registries.ITEM, ResourceLocation.parse("modid:shotgunmod"));

And now putting it all together in order:

private static ResourceKey<Item> rk = ResourceKey.create(Registries.ITEM, ResourceLocation.parse("modid:shotgunmod"));
private static final Item.Properties p = new Item.Properties().useItemDescriptionPrefix().setId(rk);
public static final RegistryObject<Item> SHOTGUN = ITEMS.register("shotgun", () -> new ShotgunItem(p));

This compiled and the mod no longer crashes. I still have more to do on it, but hopefully this will save someone hours. I welcome any feedback and if I missed some obvious modding resource or tutorial that has this information. If not, I might suggest we add it somewhere for people trying to write a mod that doesn't crash. Thank you !!!