I get this error on start-up. The server still seems to run normally though. I'm not sure if I need to do anything?

Main bit of log (full file attached):

[16:07:28] [Server thread/INFO]: Preparing start region for dimension 'world_nether'/minecraft:the_nether[16:07:28] [Server thread/INFO]: Preparing start region for dimension 'world_nether'/minecraft:the_nether[16:07:29] [Server thread/INFO]: Preparing spawn area: 0%[16:07:29] [Server thread/INFO]: Preparing spawn area: 0%[16:07:29] [Server thread/INFO]: Preparing spawn area: 89%[16:07:29] [Server thread/WARN]: Exception loading entity: net.minecraft.server.v1_15_R1.ReportedException: Loading entity NBT at net.minecraft.server.v1_15_R1.Entity.f(Entity.java:1703) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.lambda$101(EntityTypes.java:301) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.SystemUtils.a(SourceFile:341) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.a(EntityTypes.java:298) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.b(EntityTypes.java:342) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.a(EntityTypes.java:323) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkRegionLoader.loadEntities(ChunkRegionLoader.java:396) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkRegionLoader.lambda$6(ChunkRegionLoader.java:114) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.Chunk.addEntities(Chunk.java:501) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.PlayerChunkMap.lambda$32(PlayerChunkMap.java:584) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.PlayerChunkMap.lambda$30(PlayerChunkMap.java:570) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:616) ~[?:1.8.0_232] at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:591) ~[?:1.8.0_232] at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_232] at net.minecraft.server.v1_15_R1.ChunkTaskQueueSorter.b(SourceFile:58) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeTask(SourceFile:144) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeNext(SourceFile:118) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkProviderServer$a.executeNext(ChunkProviderServer.java:567) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkProviderServer.runTasks(ChunkProviderServer.java:243) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.aZ(MinecraftServer.java:926) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.executeNext(MinecraftServer.java:910) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeAll(SourceFile:103) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.executeModerately(MinecraftServer.java:887) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.loadSpawn(MinecraftServer.java:577) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:452) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:266) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:783) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_232]Caused by: java.lang.AssertionError: TRAP at net.minecraft.server.v1_15_R1.ItemStack.checkEmpty(ItemStack.java:86) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ItemStack.setCount(ItemStack.java:864) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityFireballFireball.c(SourceFile:31) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.SystemUtils.a(SourceFile:290) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityFireballFireball.b(SourceFile:31) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityFireballFireball.a(SourceFile:63) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityLargeFireball.a(EntityLargeFireball.java:54) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.Entity.f(Entity.java:1648) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] ... 30 more[16:07:29] [Server thread/WARN]: Exception loading entity: net.minecraft.server.v1_15_R1.ReportedException: Loading entity NBT at net.minecraft.server.v1_15_R1.Entity.f(Entity.java:1703) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.lambda$101(EntityTypes.java:301) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.SystemUtils.a(SourceFile:341) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.a(EntityTypes.java:298) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.b(EntityTypes.java:342) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityTypes.a(EntityTypes.java:323) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkRegionLoader.loadEntities(ChunkRegionLoader.java:396) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkRegionLoader.lambda$6(ChunkRegionLoader.java:114) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.Chunk.addEntities(Chunk.java:501) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.PlayerChunkMap.lambda$32(PlayerChunkMap.java:584) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at com.mojang.datafixers.util.Either.lambda$mapLeft$0(Either.java:162) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at com.mojang.datafixers.util.Either.mapLeft(Either.java:162) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.PlayerChunkMap.lambda$30(PlayerChunkMap.java:570) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at java.util.concurrent.CompletableFuture.uniApply(CompletableFuture.java:616) ~[?:1.8.0_232] at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:591) ~[?:1.8.0_232] at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:456) ~[?:1.8.0_232] at net.minecraft.server.v1_15_R1.ChunkTaskQueueSorter.b(SourceFile:58) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeTask(SourceFile:144) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeNext(SourceFile:118) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkProviderServer$a.executeNext(ChunkProviderServer.java:567) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ChunkProviderServer.runTasks(ChunkProviderServer.java:243) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.aZ(MinecraftServer.java:926) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.executeNext(MinecraftServer.java:910) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.IAsyncTaskHandler.executeAll(SourceFile:103) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.executeModerately(MinecraftServer.java:887) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.loadSpawn(MinecraftServer.java:577) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.a(MinecraftServer.java:452) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.DedicatedServer.init(DedicatedServer.java:266) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.MinecraftServer.run(MinecraftServer.java:783) [minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at java.lang.Thread.run(Thread.java:748) [?:1.8.0_232]Caused by: java.lang.AssertionError: TRAP at net.minecraft.server.v1_15_R1.ItemStack.checkEmpty(ItemStack.java:86) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.ItemStack.setCount(ItemStack.java:864) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityFireballFireball.c(SourceFile:31) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.SystemUtils.a(SourceFile:290) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityFireballFireball.b(SourceFile:31) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityFireballFireball.a(SourceFile:63) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.EntityLargeFireball.a(EntityLargeFireball.java:54) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] at net.minecraft.server.v1_15_R1.Entity.f(Entity.java:1648) ~[minecraft_server.jar:git-Spigot-18e2b9b-2d9db47] ... 30 more

And it continues like that for a while...!