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I got this error when I save a new dungeon or load a dungeon

[10:57:42 INFO]: BiShop_Ma issued server command: /md leave
[10:57:42 INFO]: §x[Dungeons] Cleaning up instance 'test_0'...
[10:57:42 INFO]: §x-= Saving of test processed in 54ms. =-
[10:57:42 ERROR]: null
java.util.ConcurrentModificationException: null
        at java.util.LinkedHashMap.forEach(LinkedHashMap.java:723) ~[?:?]
        at net.minecraft.world.chunk.storage.IOWorker.close(IOWorker.java:152) ~[?:?]
        at net.minecraft.world.chunk.storage.ChunkLoader.close(SourceFile:78) ~[?:?]
        at net.minecraft.world.server.ChunkManager.close(ChunkManager.java:340) ~[?:?]
        at net.minecraft.world.server.ServerChunkProvider.close(ServerChunkProvider.java:361) ~[?:?]
        at org.bukkit.craftbukkit.v1_16_R3.CraftServer.unloadWorld(CraftServer.java:1161) ~[forge:7e29f765-296df566-9fb885e8]
        at org.bukkit.Bukkit.unloadWorld(Bukkit.java:543) ~[forge:?]
        at net.playavalon.mythicdungeons.dungeons.Instance.lambda$dispose$2(Instance.java:459) ~[?:?]
        at net.playavalon.mythicdungeons.utility.ProcessTimer.run(ProcessTimer.java:9) ~[?:?]
        at net.playavalon.mythicdungeons.dungeons.Instance.lambda$dispose$4(Instance.java:459) ~[?:?]
        at net.playavalon.mythicdungeons.utility.ProcessTimer.run(ProcessTimer.java:9) ~[?:?]
        at net.playavalon.mythicdungeons.dungeons.Instance.dispose(Instance.java:397) ~[?:?]
        at net.playavalon.mythicdungeons.listeners.InstanceListener.onPlayerLeaveDungeon(InstanceListener.java:68) ~[?:?]
        at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:?]
        at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[?:?]
        at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:?]
        at java.lang.reflect.Method.invoke(Method.java:568) ~[?:?]
        at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:301) ~[forge:?]
        at co.aikar.timings.TimedEventExecutor.execute(TimedEventExecutor.java:76) ~[forge:?]
        at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:70) ~[forge:?]
        at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:593) ~[forge:?]
        at net.minecraftforge.fml.hooks.BasicEventHooks.firePlayerChangedDimensionEvent(BasicEventHooks.java:36) ~[forge:?]
        at net.minecraftforge.fml.hooks.BasicEventHooks.firePlayerChangedDimensionEvent(BasicEventHooks.java:30) ~[forge:?]
        at net.minecraft.entity.player.ServerPlayerEntity.func_200619_a(ServerPlayerEntity.java:1710) ~[?:?]
        at org.bukkit.craftbukkit.v1_16_R3.entity.CraftPlayer.teleport(CraftPlayer.java:770) ~[forge:?]
        at org.bukkit.craftbukkit.v1_16_R3.entity.CraftEntity.teleport(CraftEntity.java:644) ~[forge:?]
        at net.playavalon.mythicdungeons.dungeons.Instance.removePlayer(Instance.java:587) ~[?:?]
        at net.playavalon.mythicdungeons.MythicDungeons.onCommand(MythicDungeons.java:726) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[forge:?]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:169) ~[forge:?]
        at org.bukkit.craftbukkit.v1_16_R3.CraftServer.dispatchCommand(CraftServer.java:775) ~[forge:7e29f765-296df566-9fb885e8]
        at net.minecraft.network.play.ServerPlayNetHandler.func_147361_d(ServerPlayNetHandler.java:1833) ~[?:?]
        at net.minecraft.network.play.ServerPlayNetHandler.func_147354_a(ServerPlayNetHandler.java:1668) ~[?:?]
        at net.minecraft.network.play.client.CChatMessagePacket.func_148833_a(CChatMessagePacket.java:39) ~[?:?]
        at net.minecraft.network.play.client.CChatMessagePacket.func_148833_a(CChatMessagePacket.java:10) ~[?:?]
        at net.minecraft.network.PacketThreadUtil.lambda$ensureRunningOnSameThread$0(PacketThreadUtil.java:26) ~[?:?]
        at net.minecraft.util.concurrent.TickDelayedTask.run(SourceFile:18) ~[?:?]
        at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213166_h(ThreadTaskExecutor.java:153) ~[?:?]
        at net.minecraft.util.concurrent.RecursiveEventLoop.func_213166_h(SourceFile:23) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_213166_h(MinecraftServer.java:939) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_213166_h(MinecraftServer.java:176) ~[?:?]
        at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213168_p(ThreadTaskExecutor.java:126) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_213205_aW(MinecraftServer.java:922) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_213168_p(MinecraftServer.java:916) ~[?:?]
        at net.minecraft.util.concurrent.ThreadTaskExecutor.func_213161_c(ThreadTaskExecutor.java:136) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_213202_o(MinecraftServer.java:902) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:822) ~[?:?]
        at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:269) ~[?:?]
        at java.lang.Thread.run(Thread.java:833) ~[?:?]
[10:57:42 INFO]: §x-= Unloading world test_0 processed in 31ms. =-
[10:57:42 INFO]: §x-= Cleaning up test_0 processed in 89ms. =-