Put the summary of the bug you're having here

What I expected to happen was...:for it to work
Describe what you thought should happen here

What actually happened was...: crash with

---- Minecraft Crash Report ----
// Who set us up the TNT?

Time: 3/23/21 9:55 PM
Description: mouseClicked event handler

java.lang.IllegalStateException: failed to create a child event loop
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:88) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:58) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:47) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.MultithreadEventLoopGroup.<init>(MultithreadEventLoopGroup.java:59) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:77) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:72) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:59) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at net.minecraft.network.NetworkSystem.func_201135_f(NetworkSystem.java:46) ~[?:?] {re:classloading}
at net.minecraft.network.NetworkSystem$$Lambda$5307/1989317222.get(Unknown Source) ~[?:?] {}
at net.minecraft.util.LazyValue.func_179281_c(SourceFile:16) ~[?:?] {re:classloading}
at net.minecraft.network.NetworkSystem.func_151270_a(NetworkSystem.java:105) ~[?:?] {re:classloading}
at net.minecraft.client.Minecraft.loadWorld(Minecraft.java:1790) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_238191_a_(Minecraft.java:1681) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.gui.screen.WorldSelectionList$Entry.func_214443_e(SourceFile:407) ~[?:?] {re:classloading}
at net.minecraft.client.gui.screen.WorldSelectionList$Entry.func_214438_a(SourceFile:317) ~[?:?] {re:classloading}
at net.minecraft.client.gui.screen.WorldSelectionList$Entry.func_231044_a_(SourceFile:254) ~[?:?] {re:classloading}
at net.minecraft.client.gui.widget.list.AbstractList.func_231044_a_(AbstractList.java:309) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.gui.INestedGuiEventHandler.func_231044_a_(SourceFile:27) ~[?:?] {re:classloading}
at net.minecraft.client.MouseHelper.func_198033_b(MouseHelper.java:87) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.MouseHelper$$Lambda$4100/738772871.run(Unknown Source) ~[?:?] {}
at net.minecraft.client.gui.screen.Screen.func_231153_a_(Screen.java:427) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.MouseHelper.func_198023_a(MouseHelper.java:85) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.MouseHelper.func_228030_c_(MouseHelper.java:175) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.MouseHelper$$Lambda$4099/423639033.run(Unknown Source) ~[?:?] {}
at net.minecraft.util.concurrent.ThreadTaskExecutor.execute(ThreadTaskExecutor.java:106) ~[?:?] {re:classloading,pl:accesstransformer:B,xf:OptiFine:default}
at net.minecraft.client.MouseHelper.func_228028_b_(MouseHelper.java:174) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.MouseHelper$$Lambda$3643/721004015.invoke(Unknown Source) ~[?:?] {}
at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback(GLFWMouseButtonCallbackI.java:36) ~[lwjgl-glfw-3.2.2.jar:build 10] {}
at org.lwjgl.system.JNI.invokeV(Native Method) ~[lwjgl-3.2.2.jar:build 10] {}
at org.lwjgl.glfw.GLFW.glfwWaitEventsTimeout(GLFW.java:3174) ~[lwjgl-glfw-3.2.2.jar:build 10] {}
at com.mojang.blaze3d.systems.RenderSystem.limitDisplayFPS(SourceFile:127) ~[?:?] {re:classloading}
at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:999) [?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) [?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) [forge-1.16.5-36.1.0.jar:36.1] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$457/2036360618.call(Unknown Source) [forge-1.16.5-36.1.0.jar:36.1] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:82) [modlauncher-8.0.9.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:66) [modlauncher-8.0.9.jar:?] {}
Caused by: io.netty.channel.ChannelException: failed to open a new selector
at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:177) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoop.<init>(NioEventLoop.java:151) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:127) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:36) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:84) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
... 44 more
Caused by: java.io.IOException: Unable to establish loopback connection
at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:101) ~[?:1.8.0_51] {}
at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:68) ~[?:1.8.0_51] {}
at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_51] {}
at sun.nio.ch.PipeImpl.<init>(PipeImpl.java:170) ~[?:1.8.0_51] {}
at sun.nio.ch.SelectorProviderImpl.openPipe(SelectorProviderImpl.java:50) ~[?:1.8.0_51] {}
at java.nio.channels.Pipe.open(Pipe.java:155) ~[?:1.8.0_51] {}
at sun.nio.ch.WindowsSelectorImpl.<init>(WindowsSelectorImpl.java:127) ~[?:1.8.0_51] {}
at sun.nio.ch.WindowsSelectorProvider.openSelector(WindowsSelectorProvider.java:44) ~[?:1.8.0_51] {}
at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:175) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoop.<init>(NioEventLoop.java:151) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:127) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:36) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:84) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
... 44 more
Caused by: java.net.ConnectException: Connection refused: connect
at sun.nio.ch.Net.connect0(Native Method) ~[?:1.8.0_51] {}
at sun.nio.ch.Net.connect(Net.java:458) ~[?:1.8.0_51] {}
at sun.nio.ch.Net.connect(Net.java:450) ~[?:1.8.0_51] {}
at sun.nio.ch.SocketChannelImpl.connect(SocketChannelImpl.java:648) ~[?:1.8.0_51] {}
at java.nio.channels.SocketChannel.open(SocketChannel.java:189) ~[?:1.8.0_51] {}
at sun.nio.ch.PipeImpl$Initializer$LoopbackConnector.run(PipeImpl.java:130) ~[?:1.8.0_51] {}
at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:83) ~[?:1.8.0_51] {}
at sun.nio.ch.PipeImpl$Initializer.run(PipeImpl.java:68) ~[?:1.8.0_51] {}
at java.security.AccessController.doPrivileged(Native Method) ~[?:1.8.0_51] {}
at sun.nio.ch.PipeImpl.<init>(PipeImpl.java:170) ~[?:1.8.0_51] {}
at sun.nio.ch.SelectorProviderImpl.openPipe(SelectorProviderImpl.java:50) ~[?:1.8.0_51] {}
at java.nio.channels.Pipe.open(Pipe.java:155) ~[?:1.8.0_51] {}
at sun.nio.ch.WindowsSelectorImpl.<init>(WindowsSelectorImpl.java:127) ~[?:1.8.0_51] {}
at sun.nio.ch.WindowsSelectorProvider.openSelector(WindowsSelectorProvider.java:44) ~[?:1.8.0_51] {}
at io.netty.channel.nio.NioEventLoop.openSelector(NioEventLoop.java:175) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoop.<init>(NioEventLoop.java:151) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:127) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.newChild(NioEventLoopGroup.java:36) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:84) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
... 44 more

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– Head –
Thread: Render thread
Stacktrace:
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:88) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:58) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.util.concurrent.MultithreadEventExecutorGroup.<init>(MultithreadEventExecutorGroup.java:47) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.MultithreadEventLoopGroup.<init>(MultithreadEventLoopGroup.java:59) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:77) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:72) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at io.netty.channel.nio.NioEventLoopGroup.<init>(NioEventLoopGroup.java:59) ~[netty-all-4.1.25.Final.jar:4.1.25.Final] {}
at net.minecraft.network.NetworkSystem.func_201135_f(NetworkSystem.java:46) ~[?:?] {re:classloading}
at net.minecraft.network.NetworkSystem$$Lambda$5307/1989317222.get(Unknown Source) ~[?:?] {}
at net.minecraft.util.LazyValue.func_179281_c(SourceFile:16) ~[?:?] {re:classloading}
at net.minecraft.network.NetworkSystem.func_151270_a(NetworkSystem.java:105) ~[?:?] {re:classloading}
at net.minecraft.client.Minecraft.loadWorld(Minecraft.java:1790) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_238191_a_(Minecraft.java:1681) ~[?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.gui.screen.WorldSelectionList$Entry.func_214443_e(SourceFile:407) ~[?:?] {re:classloading}
at net.minecraft.client.gui.screen.WorldSelectionList$Entry.func_214438_a(SourceFile:317) ~[?:?] {re:classloading}
at net.minecraft.client.gui.screen.WorldSelectionList$Entry.func_231044_a_(SourceFile:254) ~[?:?] {re:classloading}
at net.minecraft.client.gui.widget.list.AbstractList.func_231044_a_(AbstractList.java:309) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.gui.INestedGuiEventHandler.func_231044_a_(SourceFile:27) ~[?:?] {re:classloading}
at net.minecraft.client.MouseHelper.func_198033_b(MouseHelper.java:87) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.MouseHelper$$Lambda$4100/738772871.run(Unknown Source) ~[?:?] {}
– Affected screen –
Details:
Screen name: net.minecraft.client.gui.screen.WorldSelectionScreen
Stacktrace:

**
Describe what happened here

Steps to Reproduce:
1. idk just happened 1 day
2.
3. ...