Hello there mod creator! Im having an issue on start up and when i checked the "crash-fml.txt" file is came with this error. Now idk what this does or how it does it. But i'm having an issue. Hopefully this can be resolved in a short timeframe, but i know how long it takes to update mods so take your time with it. Now, idk what is useful and what can be tossed so i just included everything.

Caused by 0: java.lang.reflect.InvocationTargetException
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:?] {}
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77) ~[?:?] {}
at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:?] {}
at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499) ~[?:?] {}
at java.lang.reflect.Constructor.newInstance(Constructor.java:480) ~[?:?] {}
at net.minecraftforge.fml.javafmlmod.FMLModContainer.constructMod(FMLModContainer.java:68) ~[javafmllanguage-1.20.1-47.2.0.jar%23374!/:?] {}
at net.minecraftforge.fml.ModContainer.lambda$buildTransitionHandler$10(ModContainer.java:123) ~[fmlcore-1.20.1-47.2.0.jar%23373!/:?] {re:mixin}
at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1804) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncRun.exec(CompletableFuture.java:1796) ~[?:?] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373) ~[?:?] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655) ~[?:?] {re:mixin,re:computing_frames}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622) ~[?:?] {re:mixin,re:computing_frames}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165) ~[?:?] {re:mixin}
Stacktrace:
at com.electronwill.nightconfig.core.io.ParsingException.notEnoughData(ParsingException.java:22) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.io.ReaderInput.directReadChar(ReaderInput.java:36) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.io.AbstractInput.readChar(AbstractInput.java:49) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.io.AbstractInput.readCharsUntil(AbstractInput.java:123) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.toml.TableParser.parseKey(TableParser.java:166) ~[toml-3.6.4.jar%2394!/:?] {}
at com.electronwill.nightconfig.toml.TableParser.parseDottedKey(TableParser.java:145) ~[toml-3.6.4.jar%2394!/:?] {}
at com.electronwill.nightconfig.toml.TableParser.parseNormal(TableParser.java:55) ~[toml-3.6.4.jar%2394!/:?] {}
at com.electronwill.nightconfig.toml.TomlParser.parse(TomlParser.java:44) ~[toml-3.6.4.jar%2394!/:?] {}
at com.electronwill.nightconfig.toml.TomlParser.parse(TomlParser.java:37) ~[toml-3.6.4.jar%2394!/:?] {}
at com.electronwill.nightconfig.core.io.ConfigParser.parse(ConfigParser.java:113) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.io.ConfigParser.parse(ConfigParser.java:219) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.io.ConfigParser.parse(ConfigParser.java:202) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.file.WriteSyncFileConfig.load(WriteSyncFileConfig.java:73) ~[core-3.6.4.jar%2393!/:?] {}
at com.electronwill.nightconfig.core.file.AutosaveCommentedFileConfig.load(AutosaveCommentedFileConfig.java:85) ~[core-3.6.4.jar%2393!/:?] {}
at squeek.appleskin.ModConfig.init(ModConfig.java:19) ~[appleskin-forge-mc1.20.1-2.5.1.jar%23268!/:2.5.1+mc1.20.1] {re:classloading}
at squeek.appleskin.AppleSkin.(AppleSkin.java:36) ~[appleskin-forge-mc1.20.1-2.5.1.jar%23268!/:2.5.1+mc1.20.1] {re:classloading}
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:?] {}
at jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77) ~[?:?] {}
at jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:?] {}
at java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499) ~[?:?] {}
at java.lang.reflect.Constructor.newInstance(Constructor.java:480) ~[?:?] {}
at net.minecraftforge.fml.javafmlmod.FMLModContainer.constructMod(FMLModContainer.java:68) ~[javafmllanguage-1.20.1-47.2.0.jar%23374!/:?] {}
at net.minecraftforge.fml.ModContainer.lambda$buildTransitionHandler$10(ModContainer.java:123) ~[fmlcore-1.20.1-47.2.0.jar%23373!/:?] {re:mixin}
at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1804) ~[?:?] {}
at java.util.concurrent.CompletableFuture$AsyncRun.exec(CompletableFuture.java:1796) ~[?:?] {}
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373) ~[?:?] {}
at java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182) ~[?:?] {}
at java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655) ~[?:?] {re:mixin,re:computing_frames}
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622) ~[?:?] {re:mixin,re:computing_frames}
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165) ~[?:?] {re:mixin}

Possibly looks like an error reading the config file. Try deleting config/appleskin-client.toml in your Minecraft folder and seeing if that fixes it.

Did that, came back as exit code 1

It seems like it fixed itself? After a few restarts and a few more crashes later (Ranging from exit code -1073741819 to 1, access denied errors, and other mod errors). Granted, this could be caused by something else, an updated mod perhaps? But it's now working fine. So... I guess this was (potentially) meaningless? Thanks for your assistance though.

The exit code usually doesn't mean much, it's the crash log that has the info.

Closing since there's not much I can do without being able to reproduce the problem.