HELP

[TopsyDevv]

my brother was trying to play his pixelmon world but forgot to change the installation to forge, so it defaulted to optifine (the pixelmon world was NOT on the newest version of Minecraft however optifine was) so it corrupted his world. This is very unfortunate because he put many hours into that world for it to crash whenever he tried to launch the world.

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

Time: 7/27/21 12:16 PM
Description: Exception in server tick loop

java.lang.IllegalArgumentException: ChunkNibbleArrays should be 2048 bytes not: 0
    at net.minecraft.world.chunk.NibbleArray.<init>(SourceFile:16)
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.func_75823_a(AnvilChunkLoader.java:450)
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.checkedReadChunkFromNBT__Async(AnvilChunkLoader.java:129)
    at net.minecraft.world.chunk.storage.AnvilChunkLoader.loadChunk__Async(AnvilChunkLoader.java:93)
    at net.minecraftforge.common.chunkio.ChunkIOProvider.run(ChunkIOProvider.java:70)
    at net.minecraftforge.common.chunkio.ChunkIOExecutor.syncChunkLoad(ChunkIOExecutor.java:92)
    at net.minecraft.world.gen.ChunkProviderServer.loadChunk(ChunkProviderServer.java:118)
    at net.minecraft.world.gen.ChunkProviderServer.func_186028_c(ChunkProviderServer.java:89)
    at net.minecraft.world.gen.ChunkProviderServer.func_186025_d(ChunkProviderServer.java:135)
    at net.minecraft.server.MinecraftServer.func_71222_d(MinecraftServer.java:344)
    at net.minecraft.server.integrated.IntegratedServer.func_71247_a(IntegratedServer.java:142)
    at net.minecraft.server.integrated.IntegratedServer.func_71197_b(IntegratedServer.java:156)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:486)
    at java.lang.Thread.run(Thread.java:745)

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

-- System Details --
Details:
    Minecraft Version: 1.12.2
    Operating System: Mac OS X (x86_64) version 10.11.6
    Java Version: 1.8.0_74, Oracle Corporation
    Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
    Memory: 651213248 bytes (621 MB) / 1241513984 bytes (1184 MB) up to 2147483648 bytes (2048 MB)
    JVM Flags: 7 total; -Xmx2G -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
    IntCache: cache: 2, tcache: 0, allocated: 12, tallocated: 94
    FML: MCP 9.42 Powered by Forge 14.23.5.2855 5 mods loaded, 5 mods active
    States: 'U' = Unloaded 'L' = Loaded 'C' = Constructed 'H' = Pre-initialized 'I' = Initialized 'J' = Post-initialized 'A' = Available 'D' = Disabled 'E' = Errored

    | State  | ID        | Version      | Source                              | Signature                                |
    |:------ |:--------- |:------------ |:----------------------------------- |:---------------------------------------- |
    | LCHIJA | minecraft | 1.12.2       | minecraft.jar                       | None                                     |
    | LCHIJA | mcp       | 9.42         | minecraft.jar                       | None                                     |
    | LCHIJA | FML       | 8.0.99.99    | forge-1.12.2-14.23.5.2855.jar       | e3c3d50c7c986df74c645c0ac54639741c90a557 |
    | LCHIJA | forge     | 14.23.5.2855 | forge-1.12.2-14.23.5.2855.jar       | e3c3d50c7c986df74c645c0ac54639741c90a557 |
    | LCHIJA | pixelmon  | 8.1.2        | Pixelmon-1.12.2-8.1.2-universal.jar | None                                     |

    Loaded coremods (and transformers): 
    GL info: ~~ERROR~~ RuntimeException: No OpenGL context found in the current thread.
    Profiler Position: N/A (disabled)
    Player Count: 0 / 8; []
    Type: Integrated Server (map_client.txt)
    Is Modded: Definitely; Client brand changed to 'fml,forge'