Forge Version: 36.1.18
Mods: World Pregenerator & LostCities

Issue: When pregenerating the world, the server will occasionally throw up this message, then become unresponsive. Problem does not occur when LostCities is removed.

[09:29:42] [Worker-Main-25/ERROR] [minecraft/Util]: Caught exception in thread Thread[Worker-Main-25,5,SERVER] java.lang.NullPointerException: null at net.minecraft.world.lighting.SectionLightStorage.func_215521_h(SourceFile:96) ~[?:?] {re:classloading} at net.minecraft.world.lighting.LightEngine.func_215471_c(LightEngine.java:129) ~[?:?] {re:classloading,pl:runtimedistcleaner:A} at net.minecraft.world.lighting.LevelBasedGraph.func_215469_a(LevelBasedGraph.java:122) ~[?:?] {re:classloading} at net.minecraft.world.lighting.BlockLightEngine.func_215623_a(BlockLightEngine.java:118) ~[?:?] {re:classloading} at net.minecraft.world.lighting.WorldLightManager.func_215573_a(SourceFile:41) ~[?:?] {re:classloading} at net.minecraft.world.server.ServerWorldLightManager.func_215579_a(SourceFile:135) ~[?:?] {re:mixin,re:classloading} at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(Unknown Source) ~[?:1.8.0_281] {} at java.util.stream.ReferencePipeline$Head.forEach(Unknown Source) ~[?:1.8.0_281] {} at net.minecraft.world.server.ServerWorldLightManager.func_223117_a(SourceFile:135) ~[?:?] {re:mixin,re:classloading} at net.minecraft.world.server.ServerWorldLightManager.func_215603_b(SourceFile:170) ~[?:?] {re:mixin,re:classloading} at net.minecraft.world.server.ServerWorldLightManager.func_223124_c(SourceFile:155) ~[?:?] {re:mixin,re:classloading} at net.minecraft.util.concurrent.DelegatedTaskExecutor.func_213148_e(SourceFile:94) ~[?:?] {re:classloading} at net.minecraft.util.concurrent.DelegatedTaskExecutor.func_213145_a(SourceFile:137) ~[?:?] {re:classloading} at net.minecraft.util.concurrent.DelegatedTaskExecutor.run(SourceFile:105) ~[?:?] {re:classloading} at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(Unknown Source) ~[?:1.8.0_281] {} at java.util.concurrent.ForkJoinTask.doExec(Unknown Source) ~[?:1.8.0_281] {} at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(Unknown Source) ~[?:1.8.0_281] {} at java.util.concurrent.ForkJoinPool.runWorker(Unknown Source) ~[?:1.8.0_281] {} at java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source) ~[?:1.8.0_281] {}