Here is the error:

[Thread-3526/ERROR] [EMI/]: Exception loading plugin provided by extra_mod_integrations_irons_spellbooks java.lang.NullPointerException: Cannot invoke "net.minecraft.resources.ResourceKey.registryKey()" because "key" is null at TRANSFORMER/[email protected]/net.neoforged.neoforge.common.extensions.IHolderLookupProviderExtension.holderOrThrow(IHolderLookupProviderExtension.java:24) ~[neoforge-21.1.185-universal.jar%23707!/:?] at TRANSFORMER/[email protected]+1.21.1/com.kneelawk.exmi.isns.recipe.ArcaneAnvilEmiRecipe.ofItemUpgrade(ArcaneAnvilEmiRecipe.java:112) ~[com.kneelawk.extra-mod-integrations.extra-mod-integrations-isns-neoforge-1.0.2+1.21.1.jar%231508!/:?] at TRANSFORMER/[email protected]+1.21.1/com.kneelawk.exmi.isns.recipe.ArcaneAnvilEmiRecipe.lambda$getUpgradeRecipes$6(ArcaneAnvilEmiRecipe.java:72) ~[com.kneelawk.extra-mod-integrations.extra-mod-integrations-isns-neoforge-1.0.2+1.21.1.jar%231508!/:?] at java.base/java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:197) ~[?:?] at java.base/java.util.AbstractList$RandomAccessSpliterator.tryAdvance(AbstractList.java:708) ~[?:?] at java.base/java.util.stream.StreamSpliterators$WrappingSpliterator.lambda$initPartialTraversalState$0(StreamSpliterators.java:292) ~[?:?] at java.base/java.util.stream.StreamSpliterators$AbstractWrappingSpliterator.fillBuffer(StreamSpliterators.java:206) ~[?:?] at java.base/java.util.stream.StreamSpliterators$AbstractWrappingSpliterator.doAdvance(StreamSpliterators.java:161) ~[?:?] at java.base/java.util.stream.StreamSpliterators$WrappingSpliterator.tryAdvance(StreamSpliterators.java:298) ~[?:?] at java.base/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:280) ~[?:?] at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179) ~[?:?] at java.base/java.util.Spliterators$IteratorSpliterator.tryAdvance(Spliterators.java:1950) ~[?:?] at java.base/java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:129) ~[?:?] at java.base/java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:527) ~[?:?] at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:513) ~[?:?] at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499) ~[?:?] at java.base/java.util.stream.StreamSpliterators$WrappingSpliterator.forEachRemaining(StreamSpliterators.java:310) ~[?:?] at MC-BOOTSTRAP/[email protected]/com.google.common.collect.CollectSpliterators$FlatMapSpliterator.lambda$forEachRemaining$1(CollectSpliterators.java:378) ~[guava-32.1.2-jre.jar%23111!/:?] at java.base/java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:1024) ~[?:?] at MC-BOOTSTRAP/[email protected]/com.google.common.collect.CollectSpliterators$FlatMapSpliterator.forEachRemaining(CollectSpliterators.java:374) ~[guava-32.1.2-jre.jar%23111!/:?] at java.base/java.util.stream.ReferencePipeline$Head.forEach(ReferencePipeline.java:762) ~[?:?] at TRANSFORMER/[email protected]+1.21.1/com.kneelawk.exmi.isns.ISNSIntegration.register(ISNSIntegration.java:58) ~[com.kneelawk.extra-mod-integrations.extra-mod-integrations-isns-neoforge-1.0.2+1.21.1.jar%231508!/:?] at TRANSFORMER/[email protected]+1.21.1/com.kneelawk.exmi.core.api.ExMIPlugin.register(ExMIPlugin.java:33) ~[com.kneelawk.extra-mod-integrations.extra-mod-integrations-core-neoforge-1.0.2+1.21.1.jar%231573!/:?] at TRANSFORMER/[email protected]+1.21.1/com.kneelawk.exmi.isns.ISNSPlugin.register(ISNSPlugin.java:13) ~[com.kneelawk.extra-mod-integrations.extra-mod-integrations-isns-neoforge-1.0.2+1.21.1.jar%231508!/:?] at TRANSFORMER/[email protected]+1.21.1+neoforge/dev.emi.emi.runtime.EmiReloadManager$ReloadWorker.run(EmiReloadManager.java:188) ~[emi-1.1.22+1.21.1+neoforge.jar%23850!/:?] at java.base/java.lang.Thread.run(Thread.java:1583) [?:?]

You should report this to Extra Mod Integrations instead of EMI.