Optimizing a world throws a lot of errors

Converting a 1.12 downloaded realms survival world throws a lot of errors:

[07:56:24] [Client thread/ERROR]: Something went wrong upgrading!
java.lang.NullPointerException: null
	at zj.a(SourceFile:33) ~[1.13-pre10.jar:?]
	at zj$$Lambda$1177/957997783.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$$Lambda$1006/150098236.apply(Unknown Source) ~[?:?]
	at java.util.function.Function.lambda$compose$5(Function.java:68) ~[?:1.8.0_51]
	at java.util.function.Function$$Lambda$585/137563241.apply(Unknown Source) ~[?:?]
	at java.util.function.Function.lambda$compose$5(Function.java:68) ~[?:1.8.0_51]
	at java.util.function.Function$$Lambda$585/137563241.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$$Lambda$1006/150098236.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$Instance.lambda$first$1(FunctionType.java:80) ~[datafixerupper-1.0.16.jar:?]
	at com.mojang.datafixers.FunctionType$Instance$$Lambda$1007/92737660.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$$Lambda$1006/150098236.apply(Unknown Source) ~[?:?]

[07:56:24] [Client thread/WARN]: Unsupported key: Guardian in TaggedChoiceType[id, minecraft:spawner_minecart -> ((Tag["DisplayState", NamedType["block_state", NilSave]] | NilDrop), MuType[V1486_4]), 
minecraft:giant -> ((Tag["ArmorItems", List[MuType[V1486_3]]] | NilDrop), ((Tag["HandItems", List[MuType[V1486_3]]] | NilDrop), NilSave)), 
minecraft:tnt -> NilSave, 
minecraft:xp_orb -> NilSave, 
minecraft:guardian -> ((Tag["ArmorItems", List[MuType[V1486_3]]] | NilDrop), ((Tag["HandItems", List[MuType[V1486_3]]] | NilDrop), NilSave)), 
minecraft:parrot -> NilSave, 
minecraft:tnt_minecart -> ((Tag["DisplayState", NamedType["block_state", NilSave]] | NilDrop), NilSave), 
minecraft:arrow -> ((Tag["inBlockState", NamedType["block_state", NilSave]] | NilDrop), NilSave), 
minecraft:boat -> NilSave, 
minecraft:chest_minecart -> ((Tag["DisplayState", NamedType["block_state", NilSave]] | NilDrop), ((Tag["Items", List[MuType[V1486_3]]] | NilDrop), NilSave)), 
minecraft:wither -> ((Tag["ArmorItems", List[MuType[V1486_3]]] | NilDrop), ((Tag["HandItems", List[MuType[V1486_3]]] | NilDrop), NilSave)), 
minecraft:horse -> ((Tag["ArmorItem", MuType[V1486_3]] | NilDrop), ((Tag["SaddleItem", MuType[V1486_3]] | NilDrop), ((Tag["ArmorItems", List[MuType[V1486_3]]] | NilDrop), ((Tag["HandItems", List[MuType[V1486_3]]] | NilDrop), NilSave)))), 
minecraft:trident -> ((Tag["inBlockState", NamedType["block_state", NilSave]] | NilDrop), NilSave), 
minecraft:ender_pearl -> NilSave,

[07:56:24] [Client thread/ERROR]: Something went wrong upgrading!
java.lang.NullPointerException: null
	at zj.a(SourceFile:33) ~[1.13-pre10.jar:?]
	at zj$$Lambda$1177/957997783.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$$Lambda$1006/150098236.apply(Unknown Source) ~[?:?]
	at java.util.function.Function.lambda$compose$5(Function.java:68) ~[?:1.8.0_51]
	at java.util.function.Function$$Lambda$585/137563241.apply(Unknown Source) ~[?:?]
	at java.util.function.Function.lambda$compose$5(Function.java:68) ~[?:1.8.0_51]
	at java.util.function.Function$$Lambda$585/137563241.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$$Lambda$1006/150098236.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$Instance.lambda$first$1(FunctionType.java:80) ~[datafixerupper-1.0.16.jar:?]
	at com.mojang.datafixers.FunctionType$Instance$$Lambda$1007/92737660.apply(Unknown Source) ~[?:?]
	at com.mojang.datafixers.FunctionType$$Lambda$1006/150098236.apply(Unknown Source) ~[?:?]
	at java.util.function.Function.lambda$compose$5(Function.java:68) ~[?:1.8.0_51]
	at java.util.function.Function$$Lambda$585/137563241.apply(Unknown Source) ~[?:?]
	at java.util.function.Function.lambda$compose$5(Function.java:68) ~[?:1.8.0_51]

[07:56:24] [Client thread/WARN]: Unsupported key: Guardian in TaggedChoiceType[id, minecraft:spawner_minecart -> ((Tag["DisplayState", NamedType["block_state", NilSave]] | NilDrop), MuType[V1486_4]), 
minecraft:giant -> ((Tag["ArmorItems", List[MuType[V1486_3]]] | NilDrop), ((Tag["HandItems", List[MuType[V1486_3]]] | NilDrop), NilSave)), 
minecraft:tnt -> NilSave, 
minecraft:xp_orb -> NilSave, 
minecraft:guardian -> ((Tag["ArmorItems", List[MuType[V1486_3]]] | NilDrop), ((Tag["HandItems", List[MuType[V1486_3]]] | NilDrop), NilSave)), 
minecraft:parrot -> NilSave, 
minecraft:tnt_minecart -> ((Tag["DisplayState", NamedType["block_state", NilSave]] | NilDrop), NilSave), 
minecraft:arrow -> ((Tag["inBlockState", NamedType["block_state", NilSave]] | NilDrop), NilSave), 
minecraft:boat -> NilSave, 
minecraft:chest_minecart -> ((Tag["DisplayState", NamedType["block_state", NilSave]] | NilDrop), ((Tag["Items", List[MuType[V1486_3]]] | NilDrop), NilSave)),