Works fine with "OptiFine HD U G8 preview 14" though.

I have the exact same issue.

same, and there is the stack-trace:
java.lang.NullPointerException at me.modmuss50.optifabric.mod.OptifineSetup.createMappings(OptifineSetup.java:213) at me.modmuss50.optifabric.mod.OptifineSetup.getRuntime(OptifineSetup.java:139) at me.modmuss50.optifabric.mod.OptifabricSetup.run(OptifabricSetup.java:41) at com.chocohead.mm.Plugin$$Lambda$81/1340848245.accept(Unknown Source) at java.util.ArrayList.forEach(ArrayList.java:1249) at com.chocohead.mm.Plugin.getMixins(Plugin.java:340) at org.spongepowered.asm.mixin.transformer.PluginHandle.getMixins(PluginHandle.java:128) at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:667) at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:542) at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:439) at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:415) at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:285) at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:208) at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:178) at org.spongepowered.asm.mixin.transformer.FabricMixinTransformerProxy.transformClassBytes(FabricMixinTransformerProxy.java:23) at net.fabricmc.loader.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:157) at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:150) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:348) at net.fabricmc.loader.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:45) at net.fabricmc.loader.EntrypointStorage$NewEntry.create(EntrypointStorage.java:112) at net.fabricmc.loader.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:99) at net.fabricmc.loader.EntrypointStorage.lambda$getEntrypointContainers$1(EntrypointStorage.java:184) at net.fabricmc.loader.EntrypointStorage$$Lambda$67/1023268896.get(Unknown Source) at net.fabricmc.loader.entrypoint.EntrypointContainerImpl.getEntrypoint(EntrypointContainerImpl.java:37) at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke0(EntrypointUtils.java:50) at net.fabricmc.loader.entrypoint.minecraft.hooks.EntrypointUtils.invoke(EntrypointUtils.java:36) at net.fabricmc.loader.launch.knot.Knot.init(Knot.java:137) at net.fabricmc.loader.launch.knot.KnotClient.main(KnotClient.java:28)

same

Little oversight from the 21w08b support, looks fine again with OptiFabric 1.10.2