I'm running Bukkit 1.12.1 as well as Essentials, TrainCarts, & BKCommonLib.
Here is the console:
[21:39:04] [Server thread/INFO]: [Essentials] Loading Essentials v2.0.1-b488
[21:39:04] [Server thread/INFO]: [BKCommonLib] Loading BKCommonLib v1.12.1-v3
[21:39:15] [Server thread/INFO]: [Train_Carts] Loading Train_Carts v1.12.1-v3
[21:39:15] [Server thread/INFO]: [BKCommonLib] Enabling BKCommonLib v1.12.1-v3
[21:39:15] [Server thread/INFO]: [BKCommonLib] BKCommonLib is running on CraftBukkit (git-Bukkit-8f47214) : v1_12_R1 (Minecraft 1.12.1)
[21:39:16] [Server thread/INFO]: [BKCommonLib] [Network] Now using a PlayerConnection hook to provide Packet Listener and Monitor support
[21:39:17] [Server thread/INFO]: [BKCommonLib] Now a plugin, a new server implementation tomorrow???
[21:39:17] [Server thread/INFO]: [BKCommonLib] BKCommonLib version 1.12.1-v3 (build: 211) enabled! (1.549s)
[21:39:17] [Server thread/INFO]: Preparing level "world"
[21:39:20] [Server thread/INFO]: ---- Migration of old nether folder required ----
[21:39:20] [Server thread/INFO]: Unfortunately due to the way that Minecraft implemented multiworld support in 1.6, Bukkit requires that you move your nether folder to a new location in order to operate correctly.
[21:39:20] [Server thread/INFO]: We will move this folder for you, but it will mean that you need to move it back should you wish to stop using Bukkit in the future.
[21:39:20] [Server thread/INFO]: Attempting to move world/DIM-1 to world_nether/DIM-1...
[21:39:20] [Server thread/INFO]: Success! To restore nether in the future, simply move world_nether/DIM-1 to world/DIM-1
[21:39:20] [Server thread/INFO]: ---- Migration of old nether folder complete ----
[21:39:20] [Server thread/INFO]: ---- Migration of old the_end folder required ----
[21:39:20] [Server thread/INFO]: Unfortunately due to the way that Minecraft implemented multiworld support in 1.6, Bukkit requires that you move your the_end folder to a new location in order to operate correctly.
[21:39:20] [Server thread/INFO]: We will move this folder for you, but it will mean that you need to move it back should you wish to stop using Bukkit in the future.
[21:39:20] [Server thread/INFO]: Attempting to move world/DIM1 to world_the_end/DIM1...
[21:39:20] [Server thread/INFO]: Success! To restore the_end in the future, simply move world_the_end/DIM1 to world/DIM1
[21:39:20] [Server thread/INFO]: ---- Migration of old the_end folder complete ----
[21:39:20] [Server thread/INFO]: Preparing start region for level 0 (Seed: -3961209372480882923)
[21:39:20] [Server thread/WARN]: Could not set level chunk heightmap, array length is 0 instead of 256
[21:39:20] [Server thread/ERROR]: Encountered an unexpected exception
java.lang.IllegalArgumentException: ChunkNibbleArrays should be 2048 bytes not: 0
at net.minecraft.server.v1_12_R1.NibbleArray.<init>(SourceFile:16) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:374) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:87) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.ChunkRegionLoader.loadChunk(ChunkRegionLoader.java:65) ~inecraft_server.jar:git-Bukkit-8f47214]
at org.bukkit.craftbukkit.v1_12_R1.chunkio.ChunkIOProvider.callStage1(ChunkIOProvider.java:21) ~inecraft_server.jar:git-Bukkit-8f47214]
at org.bukkit.craftbukkit.v1_12_R1.chunkio.ChunkIOProvider.callStage1(ChunkIOProvider.java:1) ~inecraft_server.jar:git-Bukkit-8f47214]
at org.bukkit.craftbukkit.v1_12_R1.util.AsynchronousExecutor.skipQueue(AsynchronousExecutor.java:336) ~inecraft_server.jar:git-Bukkit-8f47214]
at org.bukkit.craftbukkit.v1_12_R1.util.AsynchronousExecutor.getSkipQueue(AsynchronousExecutor.java:295) ~inecraft_server.jar:git-Bukkit-8f47214]
at org.bukkit.craftbukkit.v1_12_R1.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:16) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:137) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:120) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.ChunkProviderServer.getChunkAt(ChunkProviderServer.java:116) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.MinecraftServer.l(MinecraftServer.java:364) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.MinecraftServer.a(MinecraftServer.java:328) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.DedicatedServer.init(DedicatedServer.java:248) ~inecraft_server.jar:git-Bukkit-8f47214]
at net.minecraft.server.v1_12_R1.MinecraftServer.run(MinecraftServer.java:522) inecraft_server.jar:git-Bukkit-8f47214]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_144]
[21:39:20] [Server thread/ERROR]: This crash report has been saved to: /aternos/server/./crash-reports/crash-2019-01-13_21.39.20-server.txt
[21:39:20] [Server thread/INFO]: Stopping server
[21:39:20] [Server thread/INFO]: [BKCommonLib] Disabling BKCommonLib v1.12.1-v3
[21:39:20] [Server thread/INFO]: Saving players
[21:39:20] [Server thread/INFO]: Saving worlds
[21:39:20] [Server thread/INFO]: Saving chunks for level 'world'/overworld
[21:39:20] [Server thread/INFO]: Saving chunks for level 'world_nether'/the_nether
[21:39:20] [Server thread/INFO]: Saving chunks for level 'world_the_end'/the_end