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$$Lambda$90/1157943921.accept(Unknown Source)
at java.util.ArrayList.forEach(ArrayList.java:1249)
at com.chocohead.mm.Plugin.getMixins(Plugin.java:340)
at org.spongepowered.asm.mixin.transformer.PluginHandle.getMixins(PluginHandle.java:128)
at org.spongepowered.asm.mixin.transformer.MixinConfig.postInitialise(MixinConfig.java:667)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.prepareConfigs(MixinProcessor.java:542)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.select(MixinProcessor.java:439)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.checkSelect(MixinProcessor.java:415)
at org.spongepowered.asm.mixin.transformer.MixinProcessor.applyMixins(MixinProcessor.java:285)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClass(MixinTransformer.java:208)
at org.spongepowered.asm.mixin.transformer.MixinTransformer.transformClassBytes(MixinTransformer.java:178)
at org.spongepowered.asm.mixin.transformer.FabricMixinTransformerProxy.transformClassBytes(FabricMixinTransformerProxy.java:23)
at net.fabricmc.loader.launch.knot.KnotClassDelegate.getPostMixinClassByteArray(KnotClassDelegate.java:157)
at net.fabricmc.loader.launch.knot.KnotClassLoader.loadClass(KnotClassLoader.java:150)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at net.fabricmc.loader.util.DefaultLanguageAdapter.create(DefaultLanguageAdapter.java:45)
at net.fabricmc.loader.EntrypointStorage$NewEntry.create(EntrypointStorage.java:112)
at net.fabricmc.loader.EntrypointStorage$NewEntry.getOrCreate(EntrypointStorage.java:99)
at net.fabricmc.loader.EntrypointStorage.lambda$getEntrypointContainers$1(EntrypointStorage.java:184)
at net.fabricmc.loader.EntrypointStorage$$Lambda$76/2031951755.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)

You'll want to update to OptiFabric 1.10.2

Duplicate of #225