When a wolf is killed by a mob when the owner of that wolf is not online. This error message gets displayed.
[22:08:48] [Server thread/ERROR]: Could not pass event EntityDeathEvent to MyDog v0.1.3
java.lang.NullPointerException: Cannot invoke "org.bukkit.entity.Player.isOnline()" because "owner" is null
at dk.fido2603.mydog.listeners.WolfMainListener.onEntityDeathEvent(WolfMainListener.java:76) ~[MyDog-0.1.3.jar:?]
at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor25.execute(Unknown Source) ~[?:?]
at org.bukkit.plugin.EventExecutor.lambda$create$1(EventExecutor.java:75) ~[paper-api-1.18.1-R0.1-SNAPSHOT.jar:?]
at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:80) ~[paper-api-1.18.1-R0.1-SNAPSHOT.jar:git-Paper-186]
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[paper-api-1.18.1-R0.1-SNAPSHOT.jar:?]
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:628) ~[paper-api-1.18.1-R0.1-SNAPSHOT.jar:?]
at org.bukkit.craftbukkit.v1_18_R1.event.CraftEventFactory.callEntityDeathEvent(CraftEventFactory.java:868) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.world.entity.LivingEntity.dropAllDeathLoot(LivingEntity.java:1714) ~[?:?]
at net.minecraft.world.entity.LivingEntity.die(LivingEntity.java:1621) ~[?:?]
at net.minecraft.world.entity.TamableAnimal.die(TamableAnimal.java:213) ~[?:?]
at net.minecraft.world.entity.animal.Wolf.die(Wolf.java:274) ~[?:?]
at net.minecraft.world.entity.LivingEntity.hurt(LivingEntity.java:1449) ~[?:?]
at net.minecraft.world.entity.animal.Wolf.hurt(Wolf.java:323) ~[?:?]
at net.minecraft.world.entity.LivingEntity.causeFallDamage(LivingEntity.java:1885) ~[?:?]
at net.minecraft.world.level.block.Block.fallOn(Block.java:458) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.world.entity.Entity.checkFallDamage(Entity.java:1519) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.world.entity.LivingEntity.checkFallDamage(LivingEntity.java:366) ~[?:?]
at net.minecraft.world.entity.Entity.move(Entity.java:992) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.world.entity.LivingEntity.handleRelativeFrictionAndCalculateMovement(LivingEntity.java:2802) ~[?:?]
at net.minecraft.world.entity.LivingEntity.travel(LivingEntity.java:2757) ~[?:?]
at net.minecraft.world.entity.LivingEntity.aiStep(LivingEntity.java:3295) ~[?:?]
at net.minecraft.world.entity.Mob.aiStep(Mob.java:628) ~[?:?]
at net.minecraft.world.entity.AgeableMob.aiStep(AgeableMob.java:154) ~[?:?]
at net.minecraft.world.entity.animal.Animal.aiStep(Animal.java:63) ~[?:?]
at net.minecraft.world.entity.animal.Wolf.aiStep(Wolf.java:199) ~[?:?]
at net.minecraft.world.entity.LivingEntity.tick(LivingEntity.java:2905) ~[?:?]
at net.minecraft.world.entity.Mob.tick(Mob.java:396) ~[?:?]
at net.minecraft.world.entity.animal.Wolf.tick(Wolf.java:215) ~[?:?]
at net.minecraft.server.level.ServerLevel.tickNonPassenger(ServerLevel.java:1128) ~[?:?]
at net.minecraft.world.level.Level.guardEntityTick(Level.java:970) ~[?:?]
at net.minecraft.server.level.ServerLevel.lambda$tick$5(ServerLevel.java:670) ~[?:?]
at net.minecraft.world.level.entity.EntityTickList.forEach(EntityTickList.java:43) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.server.level.ServerLevel.tick(ServerLevel.java:650) ~[?:?]
at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:1607) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.server.dedicated.DedicatedServer.tickChildren(DedicatedServer.java:480) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1470) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1264) ~[paper-1.18.1.jar:git-Paper-186]
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:317) ~[paper-1.18.1.jar:git-Paper-186]
at java.lang.Thread.run(Thread.java:833) ~[?:?]
Bug reported by rosiejane through SpigotMC.