Unsetting leash holder in EntityUnleashEvent causes NPE

[datatags]

Preface: why am I doing this?

To avoid the lead dropping as an item. I'm aware Paper has an API for this, but this trick is handy because it works on any platform I've encountered, on any version since the introduction of EntityUnleashEvent, even before Paper's setDropLeash method was available.

Expected behavior

The entity to be unleashed, without the lead dropping as an item.

Observed/Actual behavior

The lead correctly does not drop, but the target entity of the EntityUnleashEvent is removed from the world entirely, and the server emits this NullPointerException: https://pastebin.com/zcPZCDki

Steps/models to reproduce

Compile and run a plugin containing the following code:

public class MyPlugin extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void onLeashBreak(EntityUnleashEvent event) {
        if (event.getEntity() instanceof LivingEntity) {
            ((LivingEntity)event.getEntity()).setLeashHolder(null);
        }
    }
}

Then, join the server and attach a lead to an entity. Ensure either that the entity cannot move, or that you are very fast. Then, move away from the entity such that the lead breaks. Observe that the entity no longer exists, and the aforementioned stack trace appears in the console.

Plugin and Datapack List

> plugins
[12:42:50 INFO]: ℹ Server Plugins (1):
[12:42:50 INFO]: Bukkit Plugins:
[12:42:50 INFO]:  - Floopin-gradle

The code of which is contained above

Paper version

> ver
[12:50:37 INFO]: Checking version, please wait...
[12:50:37 INFO]: This server is running Paper version 1.21.11-117-main@79c77f5 (2026-02-20T00:03:51Z) (Implementing API version 1.21.11-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21.11-91-a0f626b (MC: 1.21.11)

Other

[masmc05]

Why not just cancel the event? Modifying a state in the middle of a related event was always weird and prone to bugs

[datatags]

Because (up until recently, anyway) it worked on versions all the way back to the introduction of EntityUnleashEvent, as well as on certain other platforms where the event is not yet cancellable.

[mbax]

Because (up until recently, anyway) it worked on versions all the way back to the introduction of EntityUnleashEvent, as well as on certain other platforms where the event is not yet cancellable.

I mean, other platforms having no way to cancel isn't something we're going to be able to fix...

Messing with state an event is about is dangerous and unpredictable.

[datatags]

I mean, other platforms having no way to cancel isn't something we're going to be able to fix...

Messing with state an event is about is dangerous and unpredictable.

I'm not asking to fix the other platforms, my workaround has been working on this platform too until recently, so I'm asking to change this one. This seemed like an unintentional breakage and fixing it here seemed like like the shortest path to a solution, but I can look at other options if this is out of scope.

[Lulu13022002]

This doesn't really make sense the leash holder will already be unset after the event, what are you trying to do here?
Checking also only living entities would exclude boat, and spigot Leashable doesn't match paper's. If the goal is just to not drop the item then setDropLeash(false) should be enough.

[datatags]

This doesn't really make sense the leash holder will already be unset after the event, what are you trying to do here?
...
If the goal is just to not drop the item then setDropLeash(false) should be enough.

Sure, but that method isn't available in every case I'd like to cover. This workaround has historically worked on a much wider variety of Paper versions and platforms. I don't see any downsides to fixing the resulting NPE, even if there are downsides to the way I'm going about this.

Checking also only living entities would exclude boat, and spigot Leashable doesn't match paper's

My ultimate goal is more specific, the code above was just the first thing that came to mind for minimally reproducing the issue