Bukkit

Create a new world based on the flatworld generator and try to join it. You will get the following error.

[07:11:27] [Server thread/INFO]: oldUndertaker issued server command: /mw list[07:11:27] [Server thread/INFO]: oldUndertaker issued server command: /mw list
[07:11:40] [Server thread/INFO]: oldUndertaker issued server command: /mw listgens
[07:12:42] [Server thread/INFO]: oldUndertaker issued server command: /mw create wannesflat FlatLand
[07:12:42] [Server thread/INFO]: [MultiWorld] oldUndertaker: Creating new world wannesflat. Generator: FlatLand:. Seed: -2209103215204219536
[07:12:56] [Server thread/INFO]: oldUndertaker issued server command: /mw load wannesflat
[07:12:56] [Server thread/INFO]: [MultiWorld] oldUndertaker: Starting to load world wannesflat, expect some lagg!
[07:12:56] [Server thread/INFO]: -------- World Settings For [wannesflat] --------
[07:12:56] [Server thread/INFO]: Experience Merge Radius: 3.0
[07:12:56] [Server thread/INFO]: Mob Spawn Range: 6
[07:12:56] [Server thread/INFO]: Cactus Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Cane Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Melon Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Mushroom Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Pumpkin Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Sapling Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Beetroot Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Carrot Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Potato Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Wheat Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: NetherWart Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Vine Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Cocoa Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Bamboo Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: SweetBerry Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Kelp Growth Modifier: 100%
[07:12:56] [Server thread/INFO]: Entity Activation Range: An 32 / Mo 32 / Ra 48 / Mi 16 / Tiv true
[07:12:56] [Server thread/INFO]: Hopper Transfer: 8 Hopper Check: 1 Hopper Amount: 1
[07:12:56] [Server thread/INFO]: Custom Map Seeds:  Village: 10387312 Desert: 14357617 Igloo: 14357618 Jungle: 14357619 Swamp: 14357620 Monument: 10387313 Ocean: 14357621 Shipwreck: 165745295 End City: 10387313 Slime: 987234911 Bastion: 30084232 Fortress: 30084232 Mansion: 10387319 Fossil: 14357921 Portal: 34222645
[07:12:56] [Server thread/INFO]: Max TNT Explosions: 100
[07:12:56] [Server thread/INFO]: Tile Max Tick Time: 50ms Entity max Tick Time: 50ms
[07:12:56] [Server thread/INFO]: Entity Tracking Range: Pl 48 / An 48 / Mo 48 / Mi 32 / Other 64
[07:12:56] [Server thread/INFO]: Allow Zombie Pigmen to spawn from portal blocks: true
[07:12:56] [Server thread/INFO]: Item Despawn Rate: 6000
[07:12:56] [Server thread/INFO]: Item Merge Radius: 2.5
[07:12:56] [Server thread/INFO]: View Distance: 10
[07:12:56] [Server thread/INFO]: Arrow Despawn Rate: 1200 Trident Respawn Rate:1200
[07:12:56] [Server thread/INFO]: Zombie Aggressive Towards Villager: true
[07:12:56] [Server thread/INFO]: Nerfing mobs spawned from spawners: false
[07:12:56] [Server thread/INFO]: Negative index in crash report handler (20/32)
[07:12:56] [Server thread/ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'mw' in plugin MultiWorld v5.2.8
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:47) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:149) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at org.bukkit.craftbukkit.v1_16_R3.CraftServer.dispatchCommand(CraftServer.java:763) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerConnection.handleCommand(PlayerConnection.java:1788) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerConnection.c(PlayerConnection.java:1631) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerConnection.a(PlayerConnection.java:1584) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PacketPlayInChat.a(PacketPlayInChat.java:47) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerConnectionUtils.lambda$0(PlayerConnectionUtils.java:19) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.TickTask.run(SourceFile:18) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandler.executeTask(SourceFile:144) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandlerReentrant.executeTask(SourceFile:23) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandler.executeNext(SourceFile:118) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.MinecraftServer.bb(MinecraftServer.java:942) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.MinecraftServer.executeNext(MinecraftServer.java:935) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.IAsyncTaskHandler.awaitTasks(SourceFile:127) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.MinecraftServer.sleepForTick(MinecraftServer.java:919) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:851) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$0(MinecraftServer.java:164) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at java.lang.Thread.run(Thread.java:835) [?:?]
Caused by: net.minecraft.server.v1_16_R3.ReportedException: Exception generating new chunk
at net.minecraft.server.v1_16_R3.MinecraftServer.initWorld(MinecraftServer.java:470) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at org.bukkit.craftbukkit.v1_16_R3.CraftServer.createWorld(CraftServer.java:1068) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at org.bukkit.Bukkit.createWorld(Bukkit.java:533) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at multiworld.data.WorldManager.loadWorld(WorldManager.java:347) ~[?:?]
at multiworld.data.WorldHandler.loadWorld(WorldHandler.java:129) ~[?:?]
at multiworld.command.world.LoadWorldCommand.runCommand(LoadWorldCommand.java:56) ~[?:?]
at multiworld.command.Command.excute(Command.java:55) ~[?:?]
at multiworld.command.CommandMap.parseCommand(CommandMap.java:37) ~[?:?]
at multiworld.command.CommandMap.runCommand(CommandMap.java:67) ~[?:?]
at multiworld.command.Command.excute(Command.java:55) ~[?:?]
at multiworld.MultiWorldPlugin.pushCommandStack(MultiWorldPlugin.java:221) ~[?:?]
at multiworld.MultiWorldPlugin.onCommand(MultiWorldPlugin.java:206) ~[?:?]
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
... 19 more
Caused by: java.lang.UnsupportedOperationException: Custom generator multiworld.worldgen.FlatLandChunkGenerator is missing required method generateChunkData
at org.bukkit.generator.ChunkGenerator.generateChunkData(ChunkGenerator.java:117) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at org.bukkit.craftbukkit.v1_16_R3.generator.CustomChunkGenerator.buildBase(CustomChunkGenerator.java:122) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ChunkStatus.lambda$5(ChunkStatus.java:55) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ChunkStatus$d.doWork(ChunkStatus.java:233) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ChunkStatus.a(ChunkStatus.java:190) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerChunkMap.lambda$26(PlayerChunkMap.java:538) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerChunkMap.lambda$24(PlayerChunkMap.java:536) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1146) ~[?:?]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478) ~[?:?]
at net.minecraft.server.v1_16_R3.ChunkTaskQueueSorter.b(SourceFile:58) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ThreadedMailbox.e(SourceFile:94) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ThreadedMailbox.a(SourceFile:137) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ThreadedMailbox.run(SourceFile:105) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1425) ~[?:?]
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290) ~[?:?]
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020) ~[?:?]
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656) ~[?:?]
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594) ~[?:?]
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177) ~[?:?]
[07:12:56] [Worker-Main-4/FATAL]: Failed to schedule load callback for chunk [10, -15]
java.util.concurrent.CompletionException: net.minecraft.server.v1_16_R3.ReportedException: Exception generating new chunk
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314) ~[?:?]
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319) ~[?:?]
at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1155) ~[?:?]
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:478) ~[?:?]
at net.minecraft.server.v1_16_R3.ChunkTaskQueueSorter.b(SourceFile:58) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ThreadedMailbox.e(SourceFile:94) [spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ThreadedMailbox.a(SourceFile:137) [spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ThreadedMailbox.run(SourceFile:105) [spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1425) [?:?]
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290) [?:?]
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020) [?:?]
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656) [?:?]
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594) [?:?]
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177) [?:?]
Caused by: net.minecraft.server.v1_16_R3.ReportedException: Exception generating new chunk
at net.minecraft.server.v1_16_R3.PlayerChunkMap.lambda$26(PlayerChunkMap.java:551) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerChunkMap.lambda$24(PlayerChunkMap.java:536) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1146) ~[?:?]
... 11 more
Caused by: java.lang.UnsupportedOperationException: Custom generator multiworld.worldgen.FlatLandChunkGenerator is missing required method generateChunkData
at org.bukkit.generator.ChunkGenerator.generateChunkData(ChunkGenerator.java:117) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at org.bukkit.craftbukkit.v1_16_R3.generator.CustomChunkGenerator.buildBase(CustomChunkGenerator.java:122) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ChunkStatus.lambda$5(ChunkStatus.java:55) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ChunkStatus$d.doWork(ChunkStatus.java:233) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.ChunkStatus.a(ChunkStatus.java:190) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerChunkMap.lambda$26(PlayerChunkMap.java:538) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at com.mojang.datafixers.util.Either$Left.map(Either.java:38) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at net.minecraft.server.v1_16_R3.PlayerChunkMap.lambda$24(PlayerChunkMap.java:536) ~[spigot-1.16.5.jar:2991-Spigot-018b9a0-f3f3094]
at java.util.concurrent.CompletableFuture$UniCompose.tryFire(CompletableFuture.java:1146) ~[?:?]
... 11 more