Logs:
`-- Head --
Thread: Server thread
Stacktrace:
at net.minecraft.world.entity.Entity.m_20280_(Entity.java:1420) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:bookshelf.common.mixins.json:accessors.entity.AccessorEntity,pl:mixin:APP:balm.mixins.json:EntityMixin,pl:mixin:A}
at com.github.kay9.dragonmounts.dragon.ai.DragonFollowOwnerGoal.m_8045_(DragonFollowOwnerGoal.java:80) ~[dragonmounts-1.20.1-1.1.5.b3.jar%23202!/:1.20.1-1.1.5.b3] {re:classloading}
at net.minecraft.world.entity.ai.goal.WrappedGoal.m_8045_(WrappedGoal.java:27) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading}
at net.minecraft.world.entity.ai.goal.GoalSelector.m_25373_(GoalSelector.java:85) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.world.entity.Mob.m_6140_(Mob.java:760) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:mobspropertiesrandomness.mixins.json:MobMixin,pl:mixin:APP:bookshelf.common.mixins.json:accessors.entity.AccessorMob,pl:mixin:APP:immersive_armors.mixins.json:MobEntityMixin,pl:mixin:A}
at net.minecraft.world.entity.LivingEntity.m_8107_(LivingEntity.java:2548) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:puffish_skills.mixins.json:LivingEntityMixin,pl:mixin:APP:citadel.mixins.json:LivingEntityMixin,pl:mixin:APP:bookshelf.common.mixins.json:accessors.entity.AccessorLivingEntity,pl:mixin:APP:bookshelf.common.mixins.json:patches.entity.MixinLivingEntity,pl:mixin:APP:immersive_armors.mixins.json:MixinLivingEntity,pl:mixin:APP:bettercombat.mixins.json:LivingEntityAccessor,pl:mixin:APP:bettercombat.mixins.json:LivingEntityMixin,pl:mixin:APP:armordamagescale.mixins.json:LivingEntityArmorMixin,pl:mixin:APP:knavesneeds-common.mixins.json:LivingEntityMixin,pl:mixin:A}
at net.minecraft.world.entity.Mob.m_8107_(Mob.java:536) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:mobspropertiesrandomness.mixins.json:MobMixin,pl:mixin:APP:bookshelf.common.mixins.json:accessors.entity.AccessorMob,pl:mixin:APP:immersive_armors.mixins.json:MobEntityMixin,pl:mixin:A}
at net.minecraft.world.entity.AgeableMob.m_8107_(AgeableMob.java:128) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading,re:mixin}
at net.minecraft.world.entity.animal.Animal.m_8107_(Animal.java:54) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading,re:mixin}
at net.minecraft.world.entity.LivingEntity.m_8119_(LivingEntity.java:2298) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:computing_frames,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:puffish_skills.mixins.json:LivingEntityMixin,pl:mixin:APP:citadel.mixins.json:LivingEntityMixin,pl:mixin:APP:bookshelf.common.mixins.json:accessors.entity.AccessorLivingEntity,pl:mixin:APP:bookshelf.common.mixins.json:patches.entity.MixinLivingEntity,pl:mixin:APP:immersive_armors.mixins.json:MixinLivingEntity,pl:mixin:APP:bettercombat.mixins.json:LivingEntityAccessor,pl:mixin:APP:bettercombat.mixins.json:LivingEntityMixin,pl:mixin:APP:armordamagescale.mixins.json:LivingEntityArmorMixin,pl:mixin:APP:knavesneeds-common.mixins.json:LivingEntityMixin,pl:mixin:A}
at net.minecraft.world.entity.Mob.m_8119_(Mob.java:337) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:mobspropertiesrandomness.mixins.json:MobMixin,pl:mixin:APP:bookshelf.common.mixins.json:accessors.entity.AccessorMob,pl:mixin:APP:immersive_armors.mixins.json:MobEntityMixin,pl:mixin:A}
at com.github.kay9.dragonmounts.dragon.TameableDragon.m_8119_(TameableDragon.java:332) ~[dragonmounts-1.20.1-1.1.5.b3.jar%23202!/:1.20.1-1.1.5.b3] {re:classloading}
at net.minecraft.server.level.ServerLevel.m_8647_(ServerLevel.java:693) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:ServerLevelMixin,pl:mixin:APP:crafttweaker.mixins.json:common.transform.world.level.MixinServerLevel,pl:mixin:A}
at net.minecraft.world.level.Level.m_46653_(Level.java:479) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:LevelMixin,pl:mixin:A}
at net.minecraft.server.level.ServerLevel.m_184063_(ServerLevel.java:343) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:ServerLevelMixin,pl:mixin:APP:crafttweaker.mixins.json:common.transform.world.level.MixinServerLevel,pl:mixin:A}
at net.minecraft.world.level.entity.EntityTickList.m_156910_(EntityTickList.java:54) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading}
at net.minecraft.server.level.ServerLevel.m_8793_(ServerLevel.java:323) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:ServerLevelMixin,pl:mixin:APP:crafttweaker.mixins.json:common.transform.world.level.MixinServerLevel,pl:mixin:A}
-- Entity being ticked --
Details:
Entity Type: dragonmounts:dragon (com.github.kay9.dragonmounts.dragon.TameableDragon)
Entity ID: 6203
Entity Name: Aether Dragon
Entity's Exact location: 291.69, 66.00, 331.44
Entity's Block location: World: (291,66,331), Section: (at 3,2,11 in 18,4,20; chunk contains blocks 288,-64,320 to 303,319,335), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,-64,0 to 511,319,511)
Entity's Momentum: 0.03, -0.08, 0.04
Entity's Passengers: []
Entity's Vehicle: null
Stacktrace:
at net.minecraft.world.level.Level.m_46653_(Level.java:479) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:LevelMixin,pl:mixin:A}
at net.minecraft.server.level.ServerLevel.m_184063_(ServerLevel.java:343) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:ServerLevelMixin,pl:mixin:APP:crafttweaker.mixins.json:common.transform.world.level.MixinServerLevel,pl:mixin:A}
at net.minecraft.world.level.entity.EntityTickList.m_156910_(EntityTickList.java:54) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading}
at net.minecraft.server.level.ServerLevel.m_8793_(ServerLevel.java:323) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:ServerLevelMixin,pl:mixin:APP:crafttweaker.mixins.json:common.transform.world.level.MixinServerLevel,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_5703_(MinecraftServer.java:893) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at net.minecraft.server.dedicated.DedicatedServer.m_5703_(DedicatedServer.java:283) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.m_5705_(MinecraftServer.java:814) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_130011_(MinecraftServer.java:661) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_206580_(MinecraftServer.java:251) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at java.lang.Thread.run(Unknown Source) ~[?:?] {}

-- Affected level --
Details:
All players: 2 total; [ServerPlayer['P4ndorasB0x'/939, l='ServerLevel[World]', x=662.28, y=52.16, z=-1431.04], ServerPlayer['Alt_F4xx'/5298, l='ServerLevel[World]', x=297.69, y=64.00, z=324.11]]
Chunk stats: 7524
Level dimension: minecraft:overworld
Level spawn location: World: (0,75,-32), Section: (at 0,11,0 in 0,4,-2; chunk contains blocks 0,-64,-32 to 15,319,-17), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,-64,-512 to 511,319,-1)
Level time: 1608724 game time, 1608724 day time
Level name: World
Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Level weather: Rain time: 41962 (now: false), thunder time: 100834 (now: false)
Known server brands: forge
Removed feature flags:
Level was modded: true
Level storage version: 0x04ABD - Anvil
Stacktrace:
at net.minecraft.server.MinecraftServer.m_5703_(MinecraftServer.java:893) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at net.minecraft.server.dedicated.DedicatedServer.m_5703_(DedicatedServer.java:283) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.m_5705_(MinecraftServer.java:814) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_130011_(MinecraftServer.java:661) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at net.minecraft.server.MinecraftServer.m_206580_(MinecraftServer.java:251) ~[server-1.20.1-20230612.114412-srg.jar%23257!/:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,pl:mixin:APP:citadel.mixins.json:MinecraftServerMixin,pl:mixin:APP:balm.mixins.json:MinecraftServerMixin,pl:mixin:APP:crafttweaker.mixins.json:common.access.server.AccessMinecraftServer,pl:mixin:A}
at java.lang.Thread.run(Unknown Source) ~[?:?] {}
`

Steps to Reproduce:
Take the dragon on a Leash to fence and go in the nether

Description of issue:

I was able to reproduce the issue on 1.20.1 with forge 47.1.0 and dm:l 1.20.1-1.1.5.b3, so I will be testing with that. From what I've found so far this is not actually related to the lead or fence.

I'm not 100% sure at this point what the issue is, but I have an idea. I think it's that the owner is coming back null from getOwner since getPlayerByUUID is scoped to the level, the player is out of the level, and I didn't cache the owner in the goal. I'll make some changes and test again.