Issue Description: The plugin crashes on server startup

• Dynmap Version: Dynmap-3.7-beta-6-spigot.jar (for 1.21.1)
• Server Version: CraftBukkit version 4290-Spigot-5a6439b-b95736b (MC: 1.21.1) (Implementing API version 1.21.1-R0.1-SNAPSHOT)
• Server Host (if applicable): Selfhosted
• Pastebin of crashlogs or other relevant logs: https://pastebin.com/aszy4YJb
• Other Relevant Data/Screenshots: plugins installed : LuckPerms, WorldEdit, Essentials, EssentialsSpawn, EssentialsChat, WorldGuard, EssentialsAntiBuild, dynmap, WorldBorder, EssentialsProtect
• Steps to Replicate: Just install .jar in plugins folder and start server

[x] I have looked at all other issues and this is not a duplicate
[x] I have been able to replicate this

Dynmap-3.7-beta-6-spigot.jar (for 1.21.1)

There isn't yet an official Spigot release for 1.21.1, likely causing your crash.