Issue Description: Dynmap is crashing my server sometimes when i switch worlds, i assume its dynmap as i see lots of dynmap references in the crash report and happens after dynmap saves pending tile renders for the world i just left

• Dynmap Version: Dynmap-3.1-beta-3-forge-1.12.2
• Server Version: mohist server build e28b82d for 1.12.2
• Pastebin of Configuration.txt: https://pastebin.com/8iKng30G
• Server Host (if applicable): pebblehost
• Pastebin of crashlogs or other relevant logs: https://pastebin.com/pxLjbpuD
• Steps to Replicate: it only ever happens when switching worlds

[ ] I have looked at all other issues and this is not a duplicate
[ ] I have been able to replicate this < its inconsistent, only happens when switching worlds and doesnt happen all the time.

Assuming this is fixed in a later release or otherwise no longer relevant- please open a new issue if this persists.