I am making a mod and I am new so I am using this tutorial by kaupenjoe: Forge Modding Tutorials - Minecraft 1.20.X - YouTube. I made a monster called the mine creeper but it doesn't work. Whenever I tried to summon it the error message Unable to summon entity appeared. When I checked the log it said this.

[19:25:28] [Render thread/INFO] [minecraft/ChatComponent]: [System] [CHAT] Unable to summon entity
[19:25:29] [Server thread/WARN] [minecraft/EntityType]: Exception loading entity: 
java.lang.NullPointerException: Cannot invoke "net.minecraft.network.syncher.EntityDataAccessor.getId()" because "pKey" is null
    at net.minecraft.network.syncher.SynchedEntityData.define(SynchedEntityData.java:78) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.yal.yalmod.entity.custom.MineCreeperEntity.defineSynchedData(MineCreeperEntity.java:90) ~[%23194!/:?] {re:classloading}
    at net.minecraft.world.entity.Entity.<init>(Entity.java:260) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.world.entity.LivingEntity.<init>(LivingEntity.java:233) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.minecraft.world.entity.Mob.<init>(Mob.java:123) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.world.entity.PathfinderMob.<init>(PathfinderMob.java:14) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.minecraft.world.entity.monster.Monster.<init>(Monster.java:31) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.yal.yalmod.entity.custom.MineCreeperEntity.<init>(MineCreeperEntity.java:45) ~[%23194!/:?] {re:classloading}
    at net.minecraft.world.entity.EntityType.create(EntityType.java:525) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at net.minecraft.world.entity.EntityType.lambda$create$4(EntityType.java:530) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at java.util.Optional.map(Optional.java:260) ~[?:?] {}
    at net.minecraft.world.entity.EntityType.create(EntityType.java:529) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at net.minecraft.world.entity.EntityType.loadStaticEntity(EntityType.java:607) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at net.minecraft.world.entity.EntityType.loadEntityRecursive(EntityType.java:563) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at net.minecraft.server.commands.SummonCommand.createEntity(SummonCommand.java:67) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at net.minecraft.server.commands.SummonCommand.spawnEntity(SummonCommand.java:86) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at net.minecraft.server.commands.SummonCommand.lambda$register$1(SummonCommand.java:44) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,xf:fml:forge:forge_method_redirector}
    at com.mojang.brigadier.CommandDispatcher.execute(CommandDispatcher.java:264) ~[brigadier-1.1.8.jar%23134!/:?] {}
    at net.minecraft.commands.Commands.performCommand(Commands.java:257) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.minecraft.server.network.ServerGamePacketListenerImpl.performChatCommand(ServerGamePacketListenerImpl.java:1244) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.network.ServerGamePacketListenerImpl.lambda$handleChatCommand$11(ServerGamePacketListenerImpl.java:1221) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.util.thread.BlockableEventLoop.lambda$submitAsync$0(BlockableEventLoop.java:67) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?] {}
    at net.minecraft.server.TickTask.run(TickTask.java:18) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.minecraft.util.thread.BlockableEventLoop.doRunTask(BlockableEventLoop.java:156) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.util.thread.ReentrantBlockableEventLoop.doRunTask(ReentrantBlockableEventLoop.java:23) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading}
    at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:772) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.doRunTask(MinecraftServer.java:161) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.util.thread.BlockableEventLoop.pollTask(BlockableEventLoop.java:130) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.pollTaskInternal(MinecraftServer.java:755) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.pollTask(MinecraftServer.java:749) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.util.thread.BlockableEventLoop.managedBlock(BlockableEventLoop.java:139) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.waitUntilNextTick(MinecraftServer.java:735) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:667) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:251) ~[forge-1.20.1-47.0.19_mapped_parchment_2023.06.26-1.20.1.jar%23189!/:?] {re:classloading,pl:accesstransformer:B}
    at java.lang.Thread.run(Thread.java:840) ~[?:?] {}

Hi. Maybe you didn't set the attributes of the mob quite correctly. Or mod's attributes are not applied

This is my attributes line, my mob is a monster

public static AttributeSupplier.Builder createAttributes() { return Monster.createMonsterAttributes().add(Attributes.MAX_HEALTH, 8.0).add(Attributes.MOVEMENT_SPEED, 0.0).add(Attributes.ATTACK_DAMAGE, 0.0 ).add(Attributes.FOLLOW_RANGE, 10.0); }

Updated it.

public static AttributeSupplier.Builder createAttributes() {

return Monster.createMonsterAttributes()

.add(Attributes.MAX_HEALTH, 8.0)

.add(Attributes.MOVEMENT_SPEED, 0.0)

.add(Attributes.ATTACK_DAMAGE, 0.0 )

.add(Attributes.FOLLOW_RANGE, 10.0)

.add(Attributes.ARMOR, 0)

.add(Attributes.ARMOR_TOUGHNESS, 0);

}

The problem is probably with defining synched entity data.

Can I see your code