I was building a pool in my Minecraft server in Aternos, and as soon as I put down a vertical slab in the water, it completely crashed my game, and now I can't join back. Whenever I try to join my server, it turns black and then crashes my game.

This appears after I crash my game, and the crash report says:

---- Minecraft Crash Report ----
// Shall we play a game?

Time: 2025-09-14 23:42:33
Description: Tesselating block model

java.lang.NullPointerException: Cannot invoke "java.util.Map.get(Object)" because the return value of "java.util.Map.get(Object)" is null
at cjminecraft.doubleslabs.library.helpers.VerticalSlabModelHelper.getVerticalSlabModel(VerticalSlabModelHelper.java:23) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.forge.client.model.VerticalSlabBakedModel.lambda$getQuads$0(VerticalSlabBakedModel.java:60) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.api.state.IDynamicSlabStateContainer.lambda$runOnBlockStates$1(IDynamicSlabStateContainer.java:37) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.api.state.ISlabStateContainer.runOnBlockState(ISlabStateContainer.java:38) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.api.state.IDynamicSlabStateContainer.lambda$runOnBlockStates$2(IDynamicSlabStateContainer.java:37) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.common.block.entity.DynamicSlabBlockEntity.runOnStateContainers(DynamicSlabBlockEntity.java:72) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.api.state.IDynamicSlabStateContainer.runOnBlockStates(IDynamicSlabStateContainer.java:37) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.forge.client.model.VerticalSlabBakedModel.getQuads(VerticalSlabBakedModel.java:51) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at net.minecraft.client.renderer.block.ModelBlockRenderer.tesselateWithAO(ModelBlockRenderer.java:80) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraftforge.client.model.lighting.ForgeModelBlockRenderer.tesselateWithAO(ForgeModelBlockRenderer.java:65) ~[forge-1.20.1-47.4.5-universal.jar%23179!/:?] {re:classloading}
at net.minecraft.client.renderer.block.ModelBlockRenderer.tesselateBlock(ModelBlockRenderer.java:54) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.block.BlockRenderDispatcher.renderBatched(BlockRenderDispatcher.java:68) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher$RenderChunk$RebuildTask.m_234467_(ChunkRenderDispatcher.java:641) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher$RenderChunk$RebuildTask.m_5869_(ChunkRenderDispatcher.java:556) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher.m_194412_(ChunkRenderDispatcher.java:128) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760) ~[?:?] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373) ~[?:?] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622) ~[?:?] {}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165) ~[?:?] {}

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --

Thread: Render thread
Suspected Mod:
DoubleSlabs (doubleslabs), Version: 0.1.2
at TRANSFORMER/[email protected]/cjminecraft.doubleslabs.library.helpers.VerticalSlabModelHelper.getVerticalSlabModel(VerticalSlabModelHelper.java:23)
Stacktrace:
at cjminecraft.doubleslabs.library.helpers.VerticalSlabModelHelper.getVerticalSlabModel(VerticalSlabModelHelper.java:23) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
at cjminecraft.doubleslabs.forge.client.model.VerticalSlabBakedModel.lambda$getQuads$0(VerticalSlabBakedModel.java:60) ~[doubleslabs-forge-1.20.1-0.1.2.jar%23168!/:0.1.2] {re:classloading}
-- Block model being tesselated --
Details:
Block: Block{doubleslabs:vertical_slab}[axis=z,type=negative,waterlogged=true]
Block location: World: (961,96,1814), Section: (at 1,0,6 in 60,6,113; chunk contains blocks 960,-64,1808 to 975,319,1823), Region: (1,3; contains chunks 32,96 to 63,127, blocks 512,-64,1536 to 1023,319,2047)
Using AO: true
Stacktrace:
at net.minecraft.client.renderer.block.ModelBlockRenderer.tesselateBlock(ModelBlockRenderer.java:54) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.block.BlockRenderDispatcher.renderBatched(BlockRenderDispatcher.java:68) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher$RenderChunk$RebuildTask.m_234467_(ChunkRenderDispatcher.java:641) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher$RenderChunk$RebuildTask.m_5869_(ChunkRenderDispatcher.java:556) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher.m_194412_(ChunkRenderDispatcher.java:128) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760) ~[?:?] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373) ~[?:?] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622) ~[?:?] {}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165) ~[?:?] {}

-- Block being tesselated --
Details:
Block: Block{doubleslabs:vertical_slab}[axis=z,type=negative,waterlogged=true]
Block location: World: (961,96,1814), Section: (at 1,0,6 in 60,6,113; chunk contains blocks 960,-64,1808 to 975,319,1823), Region: (1,3; contains chunks 32,96 to 63,127, blocks 512,-64,1536 to 1023,319,2047)
Stacktrace:
at net.minecraft.client.renderer.block.BlockRenderDispatcher.renderBatched(BlockRenderDispatcher.java:68) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher$RenderChunk$RebuildTask.m_234467_(ChunkRenderDispatcher.java:641) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher$RenderChunk$RebuildTask.m_5869_(ChunkRenderDispatcher.java:556) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.renderer.chunk.ChunkRenderDispatcher.m_194412_(ChunkRenderDispatcher.java:128) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1760) ~[?:?] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373) ~[?:?] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622) ~[?:?] {}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165) ~[?:?] {}

-- Affected level --
Details:
All players: 1 total; [LocalPlayer['RandomGamer'/395, l='ClientLevel', x=961.69, y=97.00, z=1814.96]]
Chunk stats: 729, 408
Level dimension: minecraft:overworld
Level spawn location: World: (894,93,1826), Section: (at 14,13,2 in 55,5,114; chunk contains blocks 880,-64,1824 to 895,319,1839), Region: (1,3; contains chunks 32,96 to 63,127, blocks 512,-64,1536 to 1023,319,2047)
Level time: 3679179 game time, 160855 day time
Server brand: forge
Server type: Non-integrated multiplayer server
Stacktrace:
at net.minecraft.client.multiplayer.ClientLevel.m_6026(ClientLevel.java:455) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.m_91354_(Minecraft.java:2319) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.lambda$delayCrash$24(Minecraft.java:829) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.m_91374_(Minecraft.java:708) ~[client-1.20.1-20230612.114412-srg.jar%23174!/:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:218) ~[forge-47.4.5.jar:?] {re:classloading,pl:runtimedistcleaner:A}
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?] {}
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?] {}
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?] {}
at java.lang.reflect.Method.invoke(Method.java:569) ~[?:?] {}
at net.minecraftforge.fml.loading.targets.CommonLaunchHandler.runTarget(CommonLaunchHandler.java:111) ~[fmlloader-1.20.1-47.4.5.jar:?] {}
at net.minecraftforge.fml.loading.targets.CommonLaunchHandler.clientService(CommonLaunchHandler.java:99) ~[fmlloader-1.20.1-47.4.5.jar:?] {}
at net.minecraftforge.fml.loading.targets.CommonClientLaunchHandler.lambda$makeService$0(CommonClientLaunchHandler.java:25) ~[fmlloader-1.20.1-47.4.5.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:30) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:53) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:71) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:108) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:78) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:26) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.modlauncher.BootstrapLaunchConsumer.accept(BootstrapLaunchConsumer.java:23) ~[modlauncher-10.0.9.jar:?] {}
at cpw.mods.bootstraplauncher.BootstrapLauncher.main(BootstrapLauncher.java:141) ~[bootstraplauncher-1.1.2.jar:?] {}

-- Last reload --
Details:
Reload number: 1
Reload reason: initial
Finished: Yes
Packs: vanilla, mod_resources

Can replicate. 1.21.1 Neoforge.