Stopping a Goal can create an error

Expected behavior

No errors and no vanishing piggy would be nice 😊

Observed/Actual behavior

I recently had an issue where stopping a goal of an entity, cause a stack trace in the console (and then the entity vanished).
The goal specifically was the FollowParentGoal.

The error states that this.parent is null.
After doing some digging I came to the realization that the PaperVanillaGoal had wrapped the goal itself, and not the (NMS)WrappedGoal.
Issue here is WrappedGoal#stop stops the goal from running and also stops the goal itself.
But Paper isn't returning the WrappedGoal therefor the WrappedGoal still thinks its running.
As seen here (in PaperMobGoals):

Inside FollowParentGoal:

Inside WrappedGoal:
(This is what we're missing access to)

When Minecraft attempts to tick the goal, it first checks if it's running.
Since the WrappedGoal isn't stopped properly, it'll continue thinking its running, and therefor error.

Suggested Change:
(In PaperMobGoals return the WrappedGoal instead of the goal itself directly)

- goals.add(item.getGoal().asPaperVanillaGoal());
+ goals.add(item.asPaperVanillaGoal());

The error in question:

[12:49:13 ERROR]: Entity threw exception at world:-763.4517434552514,67.0,57948.012728867085
java.lang.NullPointerException: Cannot invoke "net.minecraft.world.entity.Entity.blockPosition()" because "entity" is null
	at net.minecraft.world.entity.ai.navigation.GroundPathNavigation.createPath(GroundPathNavigation.java:85) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.ai.navigation.PathNavigation.moveTo(PathNavigation.java:228) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.ai.goal.FollowParentGoal.tick(FollowParentGoal.java:80) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.ai.goal.WrappedGoal.tick(WrappedGoal.java:63) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.ai.goal.GoalSelector.tickRunningGoals(GoalSelector.java:128) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.ai.goal.GoalSelector.tick(GoalSelector.java:119) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.Mob.serverAiStep(Mob.java:874) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.LivingEntity.aiStep(LivingEntity.java:3396) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.Mob.aiStep(Mob.java:616) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.AgeableMob.aiStep(AgeableMob.java:148) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.animal.Animal.aiStep(Animal.java:65) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.LivingEntity.tick(LivingEntity.java:3148) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.entity.Mob.tick(Mob.java:398) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.level.ServerLevel.tickNonPassenger(ServerLevel.java:1269) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.level.Level.guardEntityTick(Level.java:1499) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.level.ServerLevel.lambda$tick$4(ServerLevel.java:817) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.world.level.entity.EntityTickList.forEach(EntityTickList.java:39) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.level.ServerLevel.tick(ServerLevel.java:799) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:1724) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1529) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1251) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:310) ~[paper-1.21.4.jar:1.21.4-100-86c6308]
	at java.base/java.lang.Thread.run(Thread.java:1570) ~[?:?]

Steps/models to reproduce

Just a little something:

private void stopPigFollowingParent(Mob mob) {
    if (mob instanceof Animals animal && !animal.isAdult()) {
        Goal<Animals> goal = Bukkit.getMobGoals().getGoal(animal, VanillaGoal.FOLLOW_PARENT);
        if (goal != null)
            goal.stop();
    }
}

Stop the goal of a baby pig following an adult pig.
You should see an error in the console and the pig will most likely vanish.

Plugin and Datapack List

[12:59:52 INFO]: Server Plugins (5):
[12:59:52 INFO]: Paper Plugins:
[12:59:52 INFO]:  - CorePlugin
[12:59:52 INFO]: Bukkit Plugins:
[12:59:52 INFO]:  - BeeConomy, PermissionsEx, StressTestBots(not enabled), Vault

> datapack list
[13:00:11 INFO]: There are 3 data pack(s) enabled: [vanilla (built-in)], [file/bukkit (world)], [paper (built-in)]
[13:00:11 INFO]: There are no more data packs available

Paper version

running Paper version 1.21.4-100-main@86c6308 (2025-01-12T17:10:40Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)

Other

No response

Start in PaperMobGoals and trace how the goal returned by Bukkit.getMobGoals().getGoal is wrapped and stopped. Reproduce with the provided baby-pig FollowParentGoal example, then verify that stopping it no longer leaves the NMS WrappedGoal running or produces the reported FollowParentGoal stack trace.