Crash when joining Essential mod private world with ViaFabricPlus active

[FemBoysAreCut3]

General Information

Dump: https://dump.viaversion.com/e3b5fb99b660a3f9d00367c4bdc62933d8dbc51c1368686feb3a1bcd2749426f

Crashed Client (who tried to join) Log: https://mclo.gs/1HUDf2V
Log of the "Hoster" Client: https://mclo.gs/pHVHW7f

Its 1:1 the Same modpack, i just copied the instance in prismlauncher

Bug Description

The client crashes immediately with a java.lang.NullPointerException when trying to join a friend's private world via the Essential Mod invitation system. The crash occurs during the RegistryDataCollector phase while handling the ClientboundFinishConfigurationPacket.

The stacktrace indicates an issue with remapping/unmapping registries during the connection handshake:
java.lang.NullPointerException: Cannot invoke "net.minecraft.core.Holder$Reference.bindComponents(net.minecraft.core.component.DataComponentMap)" because "this.element" is null

Steps to Reproduce

1. Install the "FemCraft Client" modpack (Minecraft 1.21.x/26.1.2).
2. Ensure ViaFabricPlus and Essential Mod are active.
3. Receive an invite from another player to join their private "Essential" world.
4. Click "Join".
5. The client will attempt to connect, then crash during the "Ticking screen" phase.

Expected Behavior

The client should successfully negotiate the registry handshake and connect to the private Essential world without a NullPointerException.

[Barvalg]

[Kichura]

Can you repeat this test with just viafabricplus and essential mod active? if it can be reproduced that way then something's up for sure.

[FemBoysAreCut3]

Actually, this doesn't happen if you only have those two mods installed; I just tested it. But then I wonder which other mod could be causing it? Like I said, removing ViaFabricPlus fixes it, so it's not unrelated.

[florianreuth]

The only way to find out then is by now adding back all mods one by one / or in small chunks in order to find out what mod breaks.