Issue Description: Creating a portal to the Undergarden (The Undergarden Mod) causes a crash, which mentions DynMap (see attached crash report).

Further detailed description:

• I created a new world for testing, immediately built a portal to the Undergarden and proceeded to crash upon lighting up the portal.
• Removing DynMap: The crash no longer occurs.. However:
  **
• I run a server with both DynMap and The Undergarden, and it works just fine, there are no crashes involved when attempting to light up the portal. The two mods are apparently compatible. I do not know what is causing this crash, but whatever it is, does not seem to happen all the time.
  **
  The crash happens when attempting to light up the portal, upon reloading the world the portal has correctly lit up, however.

• Dynmap Version: Dynmap-3.1-beta5-forge-1.16.3
• Server Version: forge 1.16.4 - 35.1.13
• Steps to Replicate: Please be clear in this section so we can replicate your issue
• Create a new world.
• Create a portal frame for the Undergarden portal (stone bricks).
• Use a "Catalyst" (The Undergarden, item) to light up the portal.
• Crash occurs.

[X
Crash_UG.txt
] I have looked at all other issues and this is not a duplicate
[X] I have been able to replicate this

Duplicate of #3231