When a player kills a Citizens NPC an error occurs.

[RealTriassic]

Bug Report 🐛

Plugin Version

v1.5.6

Server Version

Purpur 1.19.2

StackTrace

[06:31:44 ERROR]: Could not pass event PlayerDeathEvent to HeadDrops v1.5.6
java.lang.NullPointerException: Cannot invoke "java.lang.CharSequence.toString()" because "replacement" is null
        at java.lang.String.replace(String.java:2956) ~[?:?]
        at com.github.evancolewright.headdrops.utilities.HeadUtils.replacePlaceHolders(HeadUtils.java:38) ~[HeadDrops-1.5.6.jar:?]
        at com.github.evancolewright.headdrops.utilities.HeadUtils.createPlayerHead(HeadUtils.java:24) ~[HeadDrops-1.5.6.jar:?]
        at com.github.evancolewright.headdrops.HeadDropsListeners.onPlayerDeath(HeadDropsListeners.java:46) ~[HeadDrops-1.5.6.jar:?]
        at com.destroystokyo.paper.event.executor.asm.generated.GeneratedEventExecutor65.execute(Unknown Source) ~[?:?]
        at org.bukkit.plugin.EventExecutor.lambda$create$1(EventExecutor.java:75) ~[purpur-api-1.19.2-R0.1-SNAPSHOT.jar:?]
        at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:76) ~[purpur-api-1.19.2-R0.1-SNAPSHOT.jar:git-Purpur-1768]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[purpur-api-1.19.2-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:678) ~[purpur-api-1.19.2-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.craftbukkit.v1_19_R1.event.CraftEventFactory.callPlayerDeathEvent(CraftEventFactory.java:914) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.server.level.ServerPlayer.die(ServerPlayer.java:895) ~[?:?]
        at net.citizensnpcs.nms.v1_19_R1.entity.EntityHumanNPC.a(EntityHumanNPC.java:146) ~[Citizens-#2673.jar:?]
        at net.minecraft.world.entity.LivingEntity.hurt(LivingEntity.java:1515) ~[?:?]
        at net.minecraft.world.entity.player.Player.hurt(Player.java:1023) ~[?:?]
        at net.minecraft.server.level.ServerPlayer.hurt(ServerPlayer.java:1075) ~[?:?]
        at net.citizensnpcs.nms.v1_19_R1.entity.EntityHumanNPC.a(EntityHumanNPC.java:292) ~[Citizens-#2673.jar:?]
        at net.minecraft.world.entity.player.Player.attack(Player.java:1363) ~[?:?]
        at net.minecraft.server.level.ServerPlayer.attack(ServerPlayer.java:2176) ~[?:?]
        at net.minecraft.server.network.ServerGamePacketListenerImpl$6.a(ServerGamePacketListenerImpl.java:3086) ~[?:?]
        at net.minecraft.network.protocol.game.ServerboundInteractPacket$1.dispatch(ServerboundInteractPacket.java:24) ~[?:?]
        at net.minecraft.network.protocol.game.ServerboundInteractPacket.dispatch(ServerboundInteractPacket.java:80) ~[?:?]
        at net.minecraft.server.network.ServerGamePacketListenerImpl.handleInteract(ServerGamePacketListenerImpl.java:3006) ~[?:?]
        at net.minecraft.network.protocol.game.ServerboundInteractPacket.handle(ServerboundInteractPacket.java:67) ~[?:?]
        at net.minecraft.network.protocol.game.ServerboundInteractPacket.handle(ServerboundInteractPacket.java:12) ~[?:?]
        at net.minecraft.network.protocol.PacketUtils.lambda$ensureRunningOnSameThread$0(PacketUtils.java:51) ~[?:?]
        at net.minecraft.server.TickTask.run(TickTask.java:18) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:153) ~[?:?]
        at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:24) ~[?:?]
        at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:1388) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.server.MinecraftServer.d(MinecraftServer.java:185) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:126) ~[?:?]
        at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:1365) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:1358) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.util.thread.BlockableEventLoop.runAllTasks(BlockableEventLoop.java:114) ~[?:?]
        at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1492) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1214) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at net.minecraft.server.MinecraftServer.lambda$spin$1(MinecraftServer.java:310) ~[purpur-1.19.2.jar:git-Purpur-1768]
        at java.lang.Thread.run(Thread.java:833) ~[?:?]

Steps to Reproduce

1. Add Citizens to your server
2. Create an NPC
3. Set it to vulnerable using /npc vulnerable
4. Kill it using a sword
5. Observe.