Error while building configuration map

[zOIDAdaJulian]

Information

Server version:
This server is running Paper version git-Paper-307 (MC: 1.19.2) (Implementing API version 1.19.2-R0.1-SNAPSHOT) (Git: 476ef25)
You are running the latest version

Full output of /mv version -p:
https://paste.gg/p/anonymous/431db8d6d1db44c9ab3ac306cc97dc18

Server log:

[08:21:58] [Server thread/INFO]: .DerMobber166 issued server command: /home
[08:21:58] [Geyser player thread-3-2/WARN]: [Geyser-Spigot] Der Server verwendet Chunkhöhen die größer sind, als Bedrock akzeptieren kann! Um Probleme zu vermeiden, muss die Chunkhöhe zwischen Y-64 und Y384 für Dimension minecraft:overworld sein.
[08:21:58] [pool-144-thread-1/WARN]: [com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper] Error while building configuration map.
java.lang.NullPointerException: null key in entry: null=1
	at com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:31) ~[guava-31.0.1-jre.jar:?]
	at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:337) ~[guava-31.0.1-jre.jar:?]
	at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:447) ~[guava-31.0.1-jre.jar:?]
	at org.bukkit.craftbukkit.v1_19_R1.inventory.CraftMetaItem.serializeEnchantments(CraftMetaItem.java:1512) ~[paper-1.19.2.jar:git-Paper-307]
	at org.bukkit.craftbukkit.v1_19_R1.inventory.CraftMetaItem.serialize(CraftMetaItem.java:1435) ~[paper-1.19.2.jar:git-Paper-307]
	at org.bukkit.craftbukkit.v1_19_R1.inventory.CraftMetaItem.serialize(CraftMetaItem.java:1411) ~[paper-1.19.2.jar:git-Paper-307]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:45) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:46) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:36) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:36) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:38) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.json.JsonConfiguration.saveToString(JsonConfiguration.java:43) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at org.bukkit.configuration.file.FileConfiguration.save(FileConfiguration.java:65) ~[paper-api-1.19.2-R0.1-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource.processProfileWrite(FlatFileProfileDataSource.java:203) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource.access$300(FlatFileProfileDataSource.java:37) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource$FileWriter.call(FlatFileProfileDataSource.java:191) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource$FileWriter.call(FlatFileProfileDataSource.java:182) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]
	at java.lang.Thread.run(Thread.java:833) ~[?:?]
[08:21:58] [pool-144-thread-1/WARN]: [com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper] Error while building configuration map.
java.lang.NullPointerException: null key in entry: null=1
	at com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:31) ~[guava-31.0.1-jre.jar:?]
	at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:337) ~[guava-31.0.1-jre.jar:?]
	at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:447) ~[guava-31.0.1-jre.jar:?]
	at org.bukkit.craftbukkit.v1_19_R1.inventory.CraftMetaItem.serializeEnchantments(CraftMetaItem.java:1512) ~[paper-1.19.2.jar:git-Paper-307]
	at org.bukkit.craftbukkit.v1_19_R1.inventory.CraftMetaItem.serialize(CraftMetaItem.java:1435) ~[paper-1.19.2.jar:git-Paper-307]
	at org.bukkit.craftbukkit.v1_19_R1.inventory.CraftMetaItem.serialize(CraftMetaItem.java:1411) ~[paper-1.19.2.jar:git-Paper-307]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:45) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:46) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:36) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:36) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.buildMap(SerializationHelper.java:71) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.util.SerializationHelper.serialize(SerializationHelper.java:38) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.utils.configuration.json.JsonConfiguration.saveToString(JsonConfiguration.java:43) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at org.bukkit.configuration.file.FileConfiguration.save(FileConfiguration.java:65) ~[paper-api-1.19.2-R0.1-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource.processProfileWrite(FlatFileProfileDataSource.java:203) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource.access$300(FlatFileProfileDataSource.java:37) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource$FileWriter.call(FlatFileProfileDataSource.java:191) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at com.onarandombox.multiverseinventories.FlatFileProfileDataSource$FileWriter.call(FlatFileProfileDataSource.java:182) ~[Multiverse-Inventories-4.2.4-SNAPSHOT.jar:?]
	at java.util.concurrent.FutureTask.run(FutureTask.java:264) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) ~[?:?]
	at java.lang.Thread.run(Thread.java:833) ~[?:?]

Details

I was <able / not able> to reproduce my issue on a freshly setup and up-to-date server with the latest version of Multiverse plugins with no other plugins and with no kinds of other server or client mods.
To be honest sometimes I was able to reproduce it and sometimes not.
I really have no glue what the bug is and thats also why i made the bug report here.

Description
Everytime a player joins or switches the world an error like the above one occurs.
Sometimes they lose their inventory and enderchest and sometimes they don't. (makes absolutely no sense in my oppinion)

Steps to reproduce
I would really like to know that but suddenly the bug was here completely out of nothing.

Expected behavior
Of course the error shouldn't occur and they shouldn't lose their inventory and so on! ;D

Screenshots
Not really necessary to be honest.