Program: 27607
Project: 27607
Buggy source:
@Override public Optional<WorldProperties> createWorld(WorldCreationSettings settings){
  String worldName=settings.getWorldName();
  final Optional<World> optExisting=getWorld(worldName);
  if (optExisting.isPresent()) {
    return Optional.of(optExisting.get().getProperties());
  }
  int dim;
  AnvilSaveHandler savehandler=new AnvilSaveHandler(new File(getFolderName()),worldName,true);
  WorldInfo worldInfo=savehandler.loadWorldInfo();
  if (worldInfo != null) {
    if (!SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId()).isPresent()) {
      SpongeMod.instance.getSpongeRegistry().registerWorldProperties((WorldProperties)worldInfo);
      return Optional.of((WorldProperties)worldInfo);
    }
 else {
      return SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId());
    }
  }
 else {
    dim=DimensionManager.getNextFreeDimId();
    worldInfo=new WorldInfo((WorldSettings)(Object)settings,settings.getWorldName());
    ((WorldProperties)worldInfo).setKeepSpawnLoaded(settings.doesKeepSpawnLoaded());
    ((WorldProperties)worldInfo).setLoadOnStartup(settings.loadOnStartup());
    ((WorldProperties)worldInfo).setEnabled(settings.isEnabled());
    ((WorldProperties)worldInfo).setGeneratorType(settings.getGeneratorType());
    SpongeMod.instance.getSpongeRegistry().registerWorldProperties((WorldProperties)worldInfo);
  }
  ((IMixinWorldInfo)worldInfo).setDimensionId(dim);
  ((IMixinWorldInfo)worldInfo).setDimensionType(settings.getDimensionType());
  UUID uuid=UUID.randomUUID();
  ((IMixinWorldInfo)worldInfo).setUUID(uuid);
  SpongeMod.instance.getSpongeRegistry().registerWorldUniqueId(uuid,worldName);
  if (!DimensionManager.isDimensionRegistered(dim)) {
    DimensionManager.registerDimension(dim,((SpongeDimensionType)((WorldProperties)worldInfo).getDimensionType()).getDimensionTypeId());
  }
  savehandler.saveWorldInfoWithPlayer(worldInfo,getConfigurationManager().getHostPlayerData());
  SpongeMod.instance.getEventManager().post(SpongeEventFactory.createWorldCreate(SpongeMod.instance.getGame(),(WorldProperties)worldInfo,settings));
  return Optional.of((WorldProperties)worldInfo);
}

Fixed version (ground truth):
@Override public Optional<WorldProperties> createWorld(WorldCreationSettings settings){
  String worldName=settings.getWorldName();
  final Optional<World> optExisting=getWorld(worldName);
  if (optExisting.isPresent()) {
    return Optional.of(optExisting.get().getProperties());
  }
  int dim;
  AnvilSaveHandler savehandler=null;
  if (FMLCommonHandler.instance().getSide() == Side.CLIENT) {
    savehandler=new AnvilSaveHandler(new File(FMLCommonHandler.instance().getSavesDirectory() + File.separator + getFolderName()),worldName,true);
  }
 else {
    savehandler=new AnvilSaveHandler(new File(getFolderName()),worldName,true);
  }
  WorldInfo worldInfo=savehandler.loadWorldInfo();
  if (worldInfo != null) {
    if (!SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId()).isPresent()) {
      SpongeMod.instance.getSpongeRegistry().registerWorldProperties((WorldProperties)worldInfo);
      return Optional.of((WorldProperties)worldInfo);
    }
 else {
      return SpongeMod.instance.getSpongeRegistry().getWorldProperties(((WorldProperties)worldInfo).getUniqueId());
    }
  }
 else {
    dim=DimensionManager.getNextFreeDimId();
    worldInfo=new WorldInfo((WorldSettings)(Object)settings,settings.getWorldName());
    ((WorldProperties)worldInfo).setKeepSpawnLoaded(settings.doesKeepSpawnLoaded());
    ((WorldProperties)worldInfo).setLoadOnStartup(settings.loadOnStartup());
    ((WorldProperties)worldInfo).setEnabled(settings.isEnabled());
    ((WorldProperties)worldInfo).setGeneratorType(settings.getGeneratorType());
    SpongeMod.instance.getSpongeRegistry().registerWorldProperties((WorldProperties)worldInfo);
    SpongeMod.instance.getSpongeRegistry().registerWorldDimensionId(dim,worldName);
  }
  ((IMixinWorldInfo)worldInfo).setDimensionId(dim);
  ((IMixinWorldInfo)worldInfo).setDimensionType(settings.getDimensionType());
  UUID uuid=UUID.randomUUID();
  ((IMixinWorldInfo)worldInfo).setUUID(uuid);
  SpongeMod.instance.getSpongeRegistry().registerWorldUniqueId(uuid,worldName);
  if (!DimensionManager.isDimensionRegistered(dim)) {
    DimensionManager.registerDimension(dim,((SpongeDimensionType)((WorldProperties)worldInfo).getDimensionType()).getDimensionTypeId());
  }
  savehandler.saveWorldInfoWithPlayer(worldInfo,getConfigurationManager().getHostPlayerData());
  SpongeMod.instance.getEventManager().post(SpongeEventFactory.createWorldCreate(SpongeMod.instance.getGame(),(WorldProperties)worldInfo,settings));
  return Optional.of((WorldProperties)worldInfo);
}