[03:22:59] [Server thread/ERROR] [FML]: An Entity type com.example.structure.entity.endking.friendly.EntityFriendKing has thrown an exception trying to write state. It will not persist. Report this to the mod author net.minecraft.util.ReportedException: Saving entity NBT at net.minecraft.entity.Entity.writeToNBT(Entity.java:1772) ~[vg.class:?] at net.minecraft.entity.Entity.writeToNBTOptional(Entity.java:1654) ~[vg.class:?] at net.minecraft.world.chunk.storage.AnvilChunkLoader.writeChunkToNBT(AnvilChunkLoader.java:362) [aye.class:?] at net.minecraft.world.chunk.storage.AnvilChunkLoader.saveChunk(AnvilChunkLoader.java:174) [aye.class:?] at net.minecraft.world.gen.ChunkProviderServer.saveChunkData(ChunkProviderServer.java:202) [on.class:?] at net.minecraft.world.gen.ChunkProviderServer.saveChunks(ChunkProviderServer.java:230) [on.class:?] at net.minecraft.world.WorldServer.saveAllChunks(WorldServer.java:1007) [oo.class:?] at net.minecraft.server.MinecraftServer.saveAllWorlds(MinecraftServer.java:408) [MinecraftServer.class:?] at net.minecraft.server.MinecraftServer.tick(MinecraftServer.java:691) [MinecraftServer.class:?] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:526) [MinecraftServer.class:?] at java.lang.Thread.run(Thread.java:750) [?:1.8.0_422] Caused by: java.lang.NullPointerException at com.example.structure.entity.endking.EntityAbstractEndKing.writeEntityToNBT(EntityAbstractEndKing.java:130) ~[EntityAbstractEndKing.class:?] at net.minecraft.entity.Entity.writeToNBT(Entity.java:1743) ~[vg.class:?] ... 10 more
can provide more information if needed

yes if you could, please give the log or crash report file that has everything. helps with seeing the stacktrace. But thank you for reporting it!

This should be fixed within new updates, rewriting states along with a few non-client side checks.