Describe the bug, provide any errors
When loading a wither with a custom name (nametaged), the plugin shows an error in the server console. This seems to be an issue with the bossbar conversion. https://paste.gg/p/anonymous/011bbfcc5b404c3986e34bb34c50f0e0
This extremely spams our console as we have quite a lot of withers on our server.

How can we reproduce it?
Steps to reproduce the behavior:

1. Login on 1.8.9
2. Spawn a wither with a custom name right next to you /summon wither ~ ~ ~ {CustomName:"Custom Name Hello google.com/test",CustomNameVisible:1b}
3. Console shows error.

Expected behaviour
No error in console.

Screenshots
It does show the custom name in the bossbar anyway. The error still happens.

ViaVersion Dump:
https://dump.viaversion.com/f364d00a77f35b7511ce0e3380b43c587d697224c85b8ec9c189e3ac28f7148a

Additional server info
We use Velocity.

AuthMeVelocity, BotSentry, CommandWhitelist, EpicGuard, Geyser-Velocity, log4jfix, LuckPerms, PistonMOTD, PistonQueue, ServerUtils, Triton, VelocityTools and VelocityPluginManager

Checklist (mark with [X] to check)

• I have included a ViaVersion dump
• If applicable, I have included a paste (not a screenshot) of the error
• I have tried a build from https://ci.viaversion.com/ and the issue still persists
• I have looked at https://www.spigotmc.org/resources/viarewind-legacy-support.52924/ and it does not offer a fix for my problem

Can't reproduce with latest Via* plugins, tried a 1.9 server with a 1.8 client. This seems to get fixed by the adventure->MCStructs migration Via* plugins did a while ago, if you manage to still reproduce it with latest plugin versions, make a new issue or reopen this one.