Not being able to change the "rim" material when creating a custom delta feature for my datapack.

Whenever I try to change the "rim" material of my custom delta feature my game would get stuck on "validating datapack" whenever I try to create a world with this datapack installed.

This issue does not happen with all materials tho, If I create a delta where I make the rim out of dirt, stone, magma blocks or grass there is no problem. But when I try to create a rim made out of jungle logs, oak leaves, jungle leaves, podzol or grass blocks it would get stuck on "validating datapack"

I tried it with the blocks listed above, it could just be that I don't know how the code works and there are specific things that do and do not work. I am trying to create a custom biome where the rim is made out of leaves but haven't been able to figure out if it's a bug or a misstake caused by me.

Here is the error that the launcher logs gives when it tries to validate the datapack: 

15:02:25.261
Feature: Not a JSON object: null
15:02:25.266
Error executing task on Client java.lang.IllegalArgumentException: value already present: cio@4b74a4d at com.google.common.collect.HashBiMap.put(HashBiMap.java:287) at com.google.common.collect.HashBiMap.put(HashBiMap.java:262) at gi.a(SourceFile:64) at gi.a(SourceFile:75) at vi.b(SourceFile:66) at vi$$Lambda$3973/1356532503.apply(Unknown Source) at com.mojang.datafixers.util.Pair.mapFirst(Pair.java:64) at vi.a(SourceFile:65) at vi$$Lambda$3946/1440435946.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.mapBoth(Either.java:33) at com.mojang.serialization.DataResult.map(DataResult.java:110) at vi.a(SourceFile:65) at vh.decode(SourceFile:36) at com.mojang.serialization.Decoder.parse(Decoder.java:18) at com.mojang.serialization.codecs.FieldDecoder.decode(FieldDecoder.java:29) at com.mojang.serialization.MapCodec$1.decode(MapCodec.java:34) at com.mojang.serialization.codecs.RecordCodecBuilder$Instance$3.decode(RecordCodecBuilder.java:248) at com.mojang.serialization.codecs.RecordCodecBuilder$2.decode(RecordCodecBuilder.java:107) at com.mojang.serialization.MapDecoder.lambda$compressedDecode$0(MapDecoder.java:52) at com.mojang.serialization.MapDecoder$$Lambda$3022/1017226907.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.MapDecoder.compressedDecode(MapDecoder.java:52) at com.mojang.serialization.MapCodec$MapCodecCodec.decode(MapCodec.java:91) at com.mojang.serialization.Decoder.parse(Decoder.java:18) at com.mojang.serialization.codecs.FieldDecoder.decode(FieldDecoder.java:29) at com.mojang.serialization.MapCodec$1.decode(MapCodec.java:34) at com.mojang.serialization.MapDecoder$4.decode(MapDecoder.java:94) at com.mojang.serialization.MapCodec$1.decode(MapCodec.java:34) at com.mojang.serialization.codecs.KeyDispatchCodec.lambda$null$1(KeyDispatchCodec.java:67) at com.mojang.serialization.codecs.KeyDispatchCodec$$Lambda$3961/1892481313.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.codecs.KeyDispatchCodec.lambda$decode$2(KeyDispatchCodec.java:58) at com.mojang.serialization.codecs.KeyDispatchCodec$$Lambda$3960/643956982.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.codecs.KeyDispatchCodec.decode(KeyDispatchCodec.java:56) at com.mojang.serialization.MapCodec$3.decode(MapCodec.java:186) at com.mojang.serialization.MapDecoder.lambda$compressedDecode$0(MapDecoder.java:52) at com.mojang.serialization.MapDecoder$$Lambda$3022/1017226907.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.MapDecoder.compressedDecode(MapDecoder.java:52) at com.mojang.serialization.MapCodec$MapCodecCodec.decode(MapCodec.java:91) at com.mojang.serialization.Decoder.parse(Decoder.java:18) at vi.a(SourceFile:161) at vi.a(SourceFile:139) at vi.a(SourceFile:74) at vh.decode(SourceFile:36) at com.mojang.serialization.codecs.ListCodec.lambda$null$2(ListCodec.java:46) at com.mojang.serialization.codecs.ListCodec$$Lambda$3025/118175968.accept(Unknown Source) at com.mojang.serialization.JsonOps.lambda$getList$7(JsonOps.java:292) at com.mojang.serialization.JsonOps$$Lambda$3934/1216236294.accept(Unknown Source) at com.mojang.serialization.codecs.ListCodec.lambda$decode$4(ListCodec.java:45) at com.mojang.serialization.codecs.ListCodec$$Lambda$3024/1866608332.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.codecs.ListCodec.decode(ListCodec.java:39) at com.mojang.serialization.Decoder$3.decode(Decoder.java:77) at com.mojang.serialization.Codec$2.decode(Codec.java:71) at com.mojang.serialization.codecs.ListCodec.lambda$null$2(ListCodec.java:46) at com.mojang.serialization.codecs.ListCodec$$Lambda$3025/118175968.accept(Unknown Source) at com.mojang.serialization.JsonOps.lambda$getList$7(JsonOps.java:292) at com.mojang.serialization.JsonOps$$Lambda$3934/1216236294.accept(Unknown Source) at com.mojang.serialization.codecs.ListCodec.lambda$decode$4(ListCodec.java:45) at com.mojang.serialization.codecs.ListCodec$$Lambda$3024/1866608332.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.codecs.ListCodec.decode(ListCodec.java:39) at com.mojang.serialization.Decoder.parse(Decoder.java:18) at com.mojang.serialization.codecs.FieldDecoder.decode(FieldDecoder.java:29) at com.mojang.serialization.MapCodec$1.decode(MapCodec.java:34) at com.mojang.serialization.codecs.RecordCodecBuilder$Instance$7.decode(RecordCodecBuilder.java:409) at com.mojang.serialization.codecs.RecordCodecBuilder$Instance$7.decode(RecordCodecBuilder.java:405) at com.mojang.serialization.codecs.RecordCodecBuilder$2.decode(RecordCodecBuilder.java:107) at com.mojang.serialization.MapDecoder.lambda$compressedDecode$0(MapDecoder.java:52) at com.mojang.serialization.MapDecoder$$Lambda$3022/1017226907.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at com.mojang.serialization.MapDecoder.compressedDecode(MapDecoder.java:52) at com.mojang.serialization.MapCodec$MapCodecCodec.decode(MapCodec.java:91) at com.mojang.serialization.Decoder.parse(Decoder.java:18) at vi.a(SourceFile:161) at vi.a(SourceFile:139) at vi.a(SourceFile:105) at vi$$Lambda$3908/1214025054.apply(Unknown Source) at com.mojang.serialization.DataResult.lambda$flatMap$10(DataResult.java:138) at com.mojang.serialization.DataResult$$Lambda$636/886096689.apply(Unknown Source) at com.mojang.datafixers.util.Either$Left.map(Either.java:38) at com.mojang.serialization.DataResult.flatMap(DataResult.java:136) at vi.a(SourceFile:105) at gn.a(SourceFile:180) at gn.a(SourceFile:170) at dro.a(SourceFile:485) at dro$$Lambda$3903/2058775892.run(Unknown Source) at anw.c(SourceFile:144) at aoa.c(SourceFile:23) at anw.y(SourceFile:118) at anw.bk(SourceFile:103) at djp.e(SourceFile:987) at djp.e(SourceFile:653) at net.minecraft.client.main.Main.main(SourceFile:215)