This is the crash report:

---- Minecraft Crash Report ----
// I bet Cylons wouldn't have this problem.

Time: 5/26/21 12:56 PM
Description: Mod loading error has occurred

java.lang.Exception: Mod Loading has failed
at net.minecraftforge.fml.CrashReportExtender.dumpModLoadingCrashReport(CrashReportExtender.java:85) [?:?] {re:classloading}
at net.minecraftforge.fml.client.ClientModLoader.completeModLoading(ClientModLoader.java:188) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.lambda$null$1(Minecraft.java:508) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:immersiveengineering.mixins.json:accessors.client.MinecraftAccess,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft$$Lambda$7387/2054574724.run(Unknown Source) [?:?] {}
at net.minecraft.util.Util.func_215077_a(Util.java:430) [?:?] {re:classloading,xf:OptiFine:default}
at net.minecraft.client.Minecraft.lambda$new$2(Minecraft.java:504) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:immersiveengineering.mixins.json:accessors.client.MinecraftAccess,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft$$Lambda$6279/162566028.accept(Unknown Source) [?:?] {}
at net.minecraft.client.gui.ResourceLoadProgressGui.func_230430_a_(ResourceLoadProgressGui.java:172) [?:?] {re:classloading,xf:OptiFine:default}
at net.minecraft.client.renderer.GameRenderer.func_195458_a(GameRenderer.java:802) [?:?] {re:mixin,pl:accesstransformer:B,re:classloading,pl:accesstransformer:B,xf:OptiFine:default,pl:mixin:APP:rpgz.mixins.json:GameRendererMixin,pl:mixin:A}
at net.minecraft.client.Minecraft.func_195542_b(Minecraft.java:976) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:immersiveengineering.mixins.json:accessors.client.MinecraftAccess,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.func_99999_d(Minecraft.java:607) [?:?] {re:mixin,pl:accesstransformer:B,pl:runtimedistcleaner:A,re:classloading,pl:accesstransformer:B,pl:mixin:APP:immersiveengineering.mixins.json:accessors.client.MinecraftAccess,pl:mixin:A,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(Main.java:184) [?:?] {re:classloading,pl:runtimedistcleaner:A}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:51) [forge-1.16.5-36.1.23.jar:36.1] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$488/1066949254.call(Unknown Source) [forge-1.16.5-36.1.23.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:?] {}

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

-- Head --
Thread: Render thread
Stacktrace:
at java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011) ~[?:1.8.0_51] {}
-- MOD untamedwilds --
Details:
Mod File: untamedwilds-1.16.5-1.1.0.jar
Failure message: Untamed Wilds (untamedwilds) encountered an error during the sided_setup event phase
java.lang.NullPointerException: null
Mod Version: 1.1.0
Mod Issue URL: https://github.com/RayTrace082/untamedwilds/issues
Exception message: java.lang.NullPointerException
Stacktrace:
at java.util.concurrent.ConcurrentHashMap.putVal(ConcurrentHashMap.java:1011) ~[?:1.8.0_51] {}
at java.util.concurrent.ConcurrentHashMap.put(ConcurrentHashMap.java:1006) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.client.registry.RenderingRegistry.registerEntityRenderingHandler(RenderingRegistry.java:44) ~[?:?] {re:classloading}
at untamedwilds.init.ModEntity.registerRendering(ModEntity.java:235) ~[?:1.16.5-1.1.0] {re:classloading}
at untamedwilds.UntamedWilds.setupClient(UntamedWilds.java:70) ~[?:1.16.5-1.1.0] {re:classloading}
at untamedwilds.UntamedWilds$$Lambda$4054/425598203.accept(Unknown Source) ~[?:?] {}
at net.minecraftforge.eventbus.EventBus.doCastFilter(EventBus.java:247) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.eventbus.EventBus.lambda$addListener$11(EventBus.java:239) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.eventbus.EventBus$$Lambda$2921/1819552644.invoke(Unknown Source) ~[?:?] {}
at net.minecraftforge.eventbus.EventBus$$Lambda$2551/438049370.invoke(Unknown Source) ~[?:?] {}
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:302) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.eventbus.EventBus.post(EventBus.java:283) ~[eventbus-4.0.0.jar:?] {}
at net.minecraftforge.fml.javafmlmod.FMLModContainer.acceptEvent(FMLModContainer.java:120) ~[?:36.1] {re:classloading}
at net.minecraftforge.fml.ModContainer.lambda$buildTransitionHandler$4(ModContainer.java:121) ~[?:?] {re:classloading}
at net.minecraftforge.fml.ModContainer$$Lambda$3155/2031209493.run(Unknown Source) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1618) ~[?:1.8.0_51] {}
at java.util.concurrent.CompletableFuture$AsyncRun.exec(CompletableFuture.java:1610) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056) ~[?:1.8.0_51] {}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1689) ~[?:1.8.0_51] {re:computing_frames}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157) ~[?:1.8.0_51] {}

-- System Details --
Details:
Minecraft Version: 1.16.5
Minecraft Version ID: 1.16.5
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 4906077856 bytes (4678 MB) / 6408896512 bytes (6112 MB) up to 6408896512 bytes (6112 MB)
CPUs: 4
JVM Flags: 10 total; -Xmn128M -Xmx6102M -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xss1M -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:G1NewSizePercent=20 -XX:G1ReservePercent=20 -XX:MaxGCPauseMillis=50 -XX:G1HeapRegionSize=32M
ModLauncher: 8.0.9+86+master.3cf110c
ModLauncher launch target: fmlclient
ModLauncher naming: srg
ModLauncher services:
/mixin-0.8.2.jar mixin PLUGINSERVICE
/eventbus-4.0.0.jar eventbus PLUGINSERVICE
/forge-1.16.5-36.1.23.jar object_holder_definalize PLUGINSERVICE
/forge-1.16.5-36.1.23.jar runtime_enum_extender PLUGINSERVICE
/accesstransformers-3.0.1.jar accesstransformer PLUGINSERVICE
/forge-1.16.5-36.1.23.jar capability_inject_definalize PLUGINSERVICE
/forge-1.16.5-36.1.23.jar runtimedistcleaner PLUGINSERVICE
/mixin-0.8.2.jar mixin TRANSFORMATIONSERVICE
/OptiFine-OptiFine-1.16.5_HD_U_G8_pre2.jar OptiFine TRANSFORMATIONSERVICE
/forge-1.16.5-36.1.23.jar fml TRANSFORMATIONSERVICE
FML: 36.1
Forge: net.minecraftforge:36.1.23
FML Language Providers:
[email protected]
minecraft@1
Mod List:
forge-1.16.5-36.1.23-client.jar |Minecraft |minecraft |1.16.5 |ENQUEUE_IM|NOSIGNATURE
FarmersDelight-1.16.5-0.4.2.jar |Farmer's Delight |farmersdelight |1.16.5-0.4.2 |ENQUEUE_IM|NOSIGNATURE
crafting-dead-virus-1.16.5-1.0.0-alpha.jar |Crafting Dead Virus |craftingdeadvirus |1.0.0-alpha |ENQUEUE_IM|NOSIGNATURE
TreeChop-1.16.4-0.14.1.jar |HT's TreeChop |treechop |0.14.1 |ENQUEUE_IM|NOSIGNATURE
rpgz-0.4.4-forge.jar |RpgZ |rpgz |0.4.4-forge |ENQUEUE_IM|NOSIGNATURE
NaturalProgression-1.16.5-1.4.0.jar |Natural Progression |natural-progression |1.4.0 |ENQUEUE_IM|NOSIGNATURE
ImmersiveEngineering-1.16.5-4.2.4-134.jar |Immersive Engineering |immersiveengineering |1.16.5-4.2.4-134 |ENQUEUE_IM|44:39:94:cf:1d:8c:be:3c:7f:a9:ee:f4:1e:63:a5:ac:61:f9:c2:87:d5:5b:d9:d6:8c:b5:3e:96:5d:8e:3f:b7
locks-3.0.jar |Locks |locks |3.0 |ENQUEUE_IM|NOSIGNATURE
crafting-dead-core-1.16.5-1.4.1-alpha.jar |Crafting Dead |craftingdead |1.4.1-alpha |ENQUEUE_IM|NOSIGNATURE
jei-1.16.5-7.7.0.91.jar |Just Enough Items |jei |7.7.0.91 |ENQUEUE_IM|NOSIGNATURE
rsgauges-1.16.4-1.2.10.jar |Gauges and Switches |rsgauges |1.2.10 |ENQUEUE_IM|bf:30:76:97:e4:58:41:61:2a:f4:30:d3:8f:4c:e3:71:1d:14:c4:a1:4e:85:36:e3:1d:aa:2f:cb:22:b0:04:9b
Patchouli-1.16.4-51.jar |Patchouli |patchouli |1.16.4-51 |ENQUEUE_IM|NOSIGNATURE
journeymap-1.16.5-5.7.1.jar |Journeymap |journeymap |5.7.1 |ENQUEUE_IM|NOSIGNATURE
polymorph-forge-1.16.5-0.24.jar |Polymorph |polymorph |1.16.5-0.24 |ENQUEUE_IM|NOSIGNATURE
comforts-forge-1.16.4-4.0.1.0.jar |Comforts |comforts |1.16.4-4.0.1.0 |ENQUEUE_IM|NOSIGNATURE
citadel-1.6.3.jar |Citadel |citadel |1.6.3 |ENQUEUE_IM|NOSIGNATURE
untamedwilds-1.16.5-1.1.0.jar |Untamed Wilds |untamedwilds |1.1.0 |ERROR |NOSIGNATURE
backpacked-1.10.0-1.16.3.jar |Backpacked |backpacked |1.10.0 |ENQUEUE_IM|NOSIGNATURE
lostcities-1.16-4.0.8-beta.jar |LostCities |lostcities |1.16-4.0.8-beta |ENQUEUE_IM|NOSIGNATURE
BadMobs-1.16.5-9.0.3.jar |BadMobs |badmobs |9.0.3 |ENQUEUE_IM|eb:c4:b1:67:8b:f9:0c:db:dc:4f:01:b1:8e:61:64:39:4c:10:85:0b:a6:c4:c7:48:f0:fa:95:f2:cb:08:3a:e5
buildersaddition-1.16.5-20210421a.jar |Builders Crafts & Addition |buildersaddition |1.16.5-20210421a |ENQUEUE_IM|NOSIGNATURE
SpartanWeaponry-1.16.4-2.1.0-beta-4.jar |Spartan Weaponry |spartanweaponry |2.1.0 beta 4 |ENQUEUE_IM|NOSIGNATURE
engineersdecor-1.16.4-1.1.13.jar |Engineer's Decor |engineersdecor |1.1.13 |ENQUEUE_IM|bf:30:76:97:e4:58:41:61:2a:f4:30:d3:8f:4c:e3:71:1d:14:c4:a1:4e:85:36:e3:1d:aa:2f:cb:22:b0:04:9b
JEITweaker-1.16.5-1.0.1.15.jar |JEI Tweaker |jeitweaker |1.0.1.15 |ENQUEUE_IM|NOSIGNATURE
CraftTweaker-1.16.5-7.1.0.284.jar |CraftTweaker |crafttweaker |7.1.0.284 |ENQUEUE_IM|NOSIGNATURE
ImmersivePetroleum-1.16.5-3.2.0-3.jar |Immersive Petroleum |immersivepetroleum |3.2.0-3 |ENQUEUE_IM|NOSIGNATURE
forge-1.16.5-36.1.23-universal.jar |Forge |forge |36.1.23 |ENQUEUE_IM|22:af:21:d8:19:82:7f:93:94:fe:2b:ac:b7:e4:41:57:68:39:87:b1:a7:5c:c6:44:f9:25:74:21:14:f5:0d:90
moveplus-1.16.4-2.5.1.jar |Move Plus |moveplus |1.16.4-2.5.1 |ENQUEUE_IM|NOSIGNATURE
tl_skin_cape_forge_1.16.5-1.19.jar |TLSkinCape |tlskincape |1.19 |ENQUEUE_IM|19:f5:ce:44:81:0c:e4:22:05:5e:73:c5:a8:cd:de:f3:c8:cf:a9:b3:01:70:40:a0:ee:2d:50:7a:1c:3d:1c:8a
supplementaries-1.16.5-0.12.1.jar |Supplementaries |supplementaries |1.16.5-0.12.1 |ENQUEUE_IM|NOSIGNATURE
too_hard_for_me-1.0.0.jar |Too Hard For Me |too_hard_for_me |1.0.0 |ENQUEUE_IM|NOSIGNATURE
corpse-1.16.4-1.0.4.jar |Corpse |corpse |1.16.4-1.0.4 |ENQUEUE_IM|NOSIGNATURE
chipped-1.0.5.jar |Chipped |chipped |1.0.5 |ENQUEUE_IM|NOSIGNATURE
Crash Report UUID: 28f1f7b2-bed4-458e-8f75-dac983664796
Patchouli open book context: n/a
OptiFine Version: OptiFine_1.16.5_HD_U_G8_pre2
OptiFine Build: 20210305-002749
Render Distance Chunks: 8
Mipmaps: 4
Anisotropic Filtering: 1
Antialiasing: 0
Multitexture: false
Shaders: null
OpenGlVersion: 4.3.0 - Build 20.19.15.4963
OpenGlRenderer: Intel(R) HD Graphics 4400
OpenGlVendor: Intel
CpuCount: 4

I cannot replicate this crash. The usual suspect (Optifine) seems to work alright with Untamed Wilds

Could you share more information about how you triggered this crash? Was it while launching the game? When creating a world? When exploring the world?
Also, are you updating the mod on a pre-existing world?