Storage addItem will nicht?

Titel: `Storage.addItem() returns null for cloth sack and dedicated attached storage`

```text
Hello,

I am trying to create a loot sack from a plugin. The storage itself can be opened by a player and players can manually put items into it. However, adding valid items through the Plugin API always returns null.

I tested both approaches below. In both cases `Definitions.getItemDefinition("apple")` is valid and `Storage.addItem(...)` returns null.

Expected:
- `addItem()` returns the created item.
- The item appears in the storage.

Actual:
- `addItem()` returns null.
- The storage remains empty.
- The storage can still be opened and manually filled by a player.
```

**1. Native Storage des erzeugten `clothsack` verwenden**

```java
Objects.ObjectDefinition sackDef =
        Definitions.getObjectDefinition("clothsack");

ObjectElement sack = World.createObject(
        sackDef.id,
        0,
        sackDef.defaultcolor,
        position,
        new Quaternion(),
        new Vector3f(0.7f, 0.7f, 0.7f)
);

Storage storage = World.getStorage(sack.getGlobalID());

// storage is not null and reports 7 slots
Items.ItemDefinition apple = Definitions.getItemDefinition("apple");

Item added = storage.addItem(apple.id, 0, 1);

// added is null, storage remains empty
```

**2. Eigenen Storage erstellen und an den `clothsack` hängen**

```java
Objects.ObjectDefinition sackDef =
        Definitions.getObjectDefinition("clothsack");

ObjectElement sack = World.createObject(
        sackDef.id,
        0,
        sackDef.defaultcolor,
        position,
        new Quaternion(),
        new Vector3f(0.7f, 0.7f, 0.7f)
);

Storage storage = World.createStorage(12, false);
sack.setInfo(storage.getID());

Items.ItemDefinition apple = Definitions.getItemDefinition("apple");

Item added = storage.addItem(apple.id, 0, 1);

// added is null, storage remains empty
```

I also tried:

```java
storage.addItemToSlot(apple.id, 0, 1, 0);
```

This also returns `null`.

Delaying the insert until the sack is opened by a player did not change the behavior. The player can open the storage and manually place items into it, so the storage itself appears valid.