Null Pointer Exception in Server Thread saving chunk. Might be unrelated, but the built in server for the single player stops responding. All mobs, etc. freeze while the client still able to move around but interacting with inventories, etc. not available.

Failed to save chunk [-5, 24]
java.util.concurrent.CompletionException: java.lang.NullPointerException
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:659)
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:632)
at java.util.concurrent.CompletableFuture$Completion.run(CompletableFuture.java:442)
at net.minecraft.world.server.ChunkManager.func_223155_b(ChunkManager.java:488)
at net.minecraft.world.server.ChunkManager.func_219204_a(ChunkManager.java:460)
at net.minecraft.world.server.ServerChunkProvider.func_217207_a(ServerChunkProvider.java:323)
at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:333)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:851)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:787)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:118)
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:642)
at net.minecraft.server.MinecraftServer.lambda$startServer$0(MinecraftServer.java:232)
at net.minecraft.server.MinecraftServer$$Lambda$29897/1719232273.run(Unknown Source)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
at appeng.hooks.TickHandler.onUnloadChunk(TickHandler.java:180)
at appeng.hooks.TickHandler$$Lambda$14481/2028274020.accept(Unknown Source)
at net.minecraftforge.eventbus.EventBus.doCastFilter(EventBus.java:247)
at net.minecraftforge.eventbus.EventBus.lambda$addListener$11(EventBus.java:239)
at net.minecraftforge.eventbus.EventBus$$Lambda$2990/1758225177.invoke(Unknown Source)
at net.minecraftforge.eventbus.EventBus$$Lambda$2621/2041774190.invoke(Unknown Source)
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:302)
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283)
at sun.reflect.GeneratedMethodAccessor120.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.optifine.reflect.Reflector.call(Reflector.java:1084)
at net.optifine.reflect.Reflector.postForgeBusEvent(Reflector.java:1357)
at net.optifine.reflect.Reflector.postForgeBusEvent(Reflector.java:1343)
at net.minecraft.world.server.ChunkManager.lambda$scheduleSave$10(ChunkManager.java:511)
at net.minecraft.world.server.ChunkManager$$Lambda$32882/387864167.accept(Unknown Source)
at java.util.concurrent.CompletableFuture.uniAccept(CompletableFuture.java:656)
... 13 more

• Environment:
• appliedenergistics2-8.3.0-alpha.2.jar
• Custom Modpack under development
• 1.16.5 forge version 36.0.42 with OptiFine 1.16.5_HD_U_G7
• Forced crash report: https://pastebin.com/D5ZaVhJc

Please try it without optifine or other "performance" enhancing mods.

Also we cannot support modpacks, which use other mods changing our. Which can result in all sorts of bugs.

Reproduced with Forge version 36.0.58 and ONLY AE2 installed. See below:

[20:15:52] [Server thread/ERROR] [ne.mi.ev.EventBus/EVENTBUS]: Exception caught during firing event: null
Index: 3
Listeners:
0: NORMAL
1: ASM: net.minecraftforge.common.ForgeInternalHandler@4aad0e00 onChunkUnload(Lnet/minecraftforge/event/world/ChunkEvent$Unload;)V
2: ASM: class net.minecraftforge.client.model.ModelDataManager onChunkUnload(Lnet/minecraftforge/event/world/ChunkEvent$Unload;)V
3: net.minecraftforge.eventbus.EventBus$$Lambda$2959/676752045@39922326
java.lang.NullPointerException
at appeng.hooks.TickHandler.onUnloadChunk(TickHandler.java:180)
at net.minecraftforge.eventbus.EventBus.doCastFilter(EventBus.java:247)
at net.minecraftforge.eventbus.EventBus.lambda$addListener$11(EventBus.java:239)
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:302)
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283)
at net.minecraft.world.server.ChunkManager.func_219185_a(ChunkManager.java:403)
at java.util.concurrent.CompletableFuture.uniAccept(Unknown Source)
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(Unknown Source)
at java.util.concurrent.CompletableFuture$Completion.run(Unknown Source)
at net.minecraft.world.server.ChunkManager.func_223155_b(ChunkManager.java:388)
at net.minecraft.world.server.ChunkManager.func_219204_a(ChunkManager.java:366)
at net.minecraft.world.server.ServerChunkProvider.func_217207_a(ServerChunkProvider.java:323)
at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:333)
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:851)
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:787)
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:78)
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:642)
at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:232)
at java.lang.Thread.run(Unknown Source)

[20:15:52] [Server thread/ERROR] [minecraft/ChunkManager]: Failed to save chunk [-11, 58]
java.util.concurrent.CompletionException: java.lang.NullPointerException
at java.util.concurrent.CompletableFuture.encodeThrowable(Unknown Source) ~[?:1.8.0_171] {}
at java.util.concurrent.CompletableFuture.completeThrowable(Unknown Source) ~[?:1.8.0_171] {}
at java.util.concurrent.CompletableFuture.uniAccept(Unknown Source) ~[?:1.8.0_171] {}
at java.util.concurrent.CompletableFuture$UniAccept.tryFire(Unknown Source) ~[?:1.8.0_171] {}
at java.util.concurrent.CompletableFuture$Completion.run(Unknown Source) ~[?:1.8.0_171] {}
at net.minecraft.world.server.ChunkManager.func_223155_b(ChunkManager.java:388) ~[?:?] {re:classloading}
at net.minecraft.world.server.ChunkManager.func_219204_a(ChunkManager.java:366) ~[?:?] {re:classloading}
at net.minecraft.world.server.ServerChunkProvider.func_217207_a(ServerChunkProvider.java:323) ~[?:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.world.server.ServerWorld.func_72835_b(ServerWorld.java:333) ~[?:?] {re:classloading}
at net.minecraft.server.MinecraftServer.func_71190_q(MinecraftServer.java:851) ~[?:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:787) ~[?:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:78) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.server.MinecraftServer.func_240802_v_(MinecraftServer.java:642) ~[?:?] {re:classloading,pl:accesstransformer:B}
at net.minecraft.server.MinecraftServer.func_240783_a_(MinecraftServer.java:232) ~[?:?] {re:classloading,pl:accesstransformer:B}
at java.lang.Thread.run(Unknown Source) [?:1.8.0_171] {}
Caused by: java.lang.NullPointerException
at appeng.hooks.TickHandler.onUnloadChunk(TickHandler.java:180) ~[?:8.3.0-alpha.2] {re:classloading}
at net.minecraftforge.eventbus.EventBus.doCastFilter(EventBus.java:247) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.eventbus.EventBus.lambda$addListener$11(EventBus.java:239) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:302) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283) ~[eventbus-4.0.0.jar:?] {}
at net.minecraft.world.server.ChunkManager.func_219185_a(ChunkManager.java:403) ~[?:?] {re:classloading}
... 13 more

Forced crash log:
https://pastebin.com/yLEUGr8Q