I'm having intermittent errors when trying to set the biome in a world. Steps to reproduce: Fresh server, startup first time allow plugins to generate worlds Stop server Start server again Immediately login, use plugin to load a custom schematic and place the blocks into a world that the player is NOT in. The methods calls World#setBiome(x,z); NPE setting biome (see below). Try again after pulling hair out, and it works without issues. Code (Text): Caused by: java.lang.NullPointerException at org.bukkit.craftbukkit.v1_15_R1.CraftWorld.setBiome(CraftWorld.java:942) ~[spigot.jar:git-Spigot-c574e08-708be69] at me.eccentric_nz.TARDIS.builders.TARDISBuilderInner.buildInner(TARDISBuilderInner.java:206) ~[?:?] I have checked that the world is not null and the chunk is not null. I preload the chunk(s) before placing blocks. I also realise that World#setBiome(x, z) is now deprecated, but the method just calls the non-deprecated one anyway so shouldn't be an issue. The line that throws the error is (in CraftWorld.java): Code (Text): chunk.getBiomeIndex().setBiome(x >> 2, y >> 2, z >> 2, bb); Which to me would mean that the biome index is null. I can catch the NPE when it occurs and the schematic will continue to be placed, but that's a pretty inelegant solution. Does anybody know what the heart of the problem might be?