Task #27 for RB generated an exception

[wingzero54]

[01:10:14] [Server thread/WARN]: [RealisticBiomes] Task #27 for RealisticBiomes v2.1.1 generated an exception
java.util.ConcurrentModificationException: null
at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1208) ~[?:?]
at java.util.TreeMap$KeyIterator.next(TreeMap.java:1262) ~[?:?]
at vg.civcraft.mc.civmodcore.util.progress.ProgressTracker.processItems(ProgressTracker.java:34) ~[?:?]
at com.untamedears.realisticbiomes.model.RBChunkCache.updateState(RBChunkCache.java:70) ~[?:?]
at vg.civcraft.mc.civmodcore.util.progress.ProgressTracker.processItems(ProgressTracker.java:40) ~[?:?]
at com.untamedears.realisticbiomes.PlantProgressManager.processUpdates(PlantProgressManager.java:25) ~[?:?]
at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftTask.run(CraftTask.java:99) ~[patched_1.16.4.jar:git-Paper-355]
at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:468) ~[patched_1.16.4.jar:git-Paper-355]
at net.minecraft.server.v1_16_R3.MinecraftServer.b(MinecraftServer.java:1294) ~[patched_1.16.4.jar:git-Paper-355]
at net.minecraft.server.v1_16_R3.DedicatedServer.b(DedicatedServer.java:377) ~[patched_1.16.4.jar:git-Paper-355]
at net.minecraft.server.v1_16_R3.MinecraftServer.a(MinecraftServer.java:1209) ~[patched_1.16.4.jar:git-Paper-355]
at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:997) ~[patched_1.16.4.jar:git-Paper-355]
at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$a$0(MinecraftServer.java:178) ~[patched_1.16.4.jar:git-Paper-355]
at java.lang.Thread.run(Thread.java:834) [?:?]