Issue description:
crashed with a datapack which generate a sky island dimension.
only mod or only dim wont cause the crash

If crashed, link to crash report (use a site such as pastebin):
https://pastebin.com/YhJqX9w9

Versions:

• Minecraft: 1.16.5
• Forge: 36.0.43
• Mantle: 1.6.79
• Tinkers Construct: 3.0.1.21

Can it be reproduced with just Tinkers Construct? If not, list the other mods required to reproduce the issue.
architectury
ftb-gui-library
jei
OpenLoader
jeiintegration
Jade

it seems occour on any new dims with datapack that generate structures in the sky

First off, that is an incomplete crash report, provide the entire crash report.

Second off, there is no official 1.16 build. As a result its hard to verify you have a properly made build.

Finally, pretty sure this is a bug with dimension mods, as our logic works perfect with vanilla. Looks like they are creating a dimension with an immutable list instead of a mutable one.

First off, that is an incomplete crash report, provide the entire crash report.

Second off, there is no official 1.16 build. As a result its hard to verify you have a properly made build.

Finally, pretty sure this is a bug with dimension mods, as our logic works perfect with vanilla. Looks like they are creating a dimension with an immutable list instead of a mutable one.

https://pastebin.com/GzCWrBL3
the build is from:https://dvs1.progwml6.com/files/maven/slimeknights/tconstruct/TConstruct/
I made a new client,only have mantle,tconstruct and used the datapack with skyland dimension
I tryed
with datapack:
superflat void dim,OK
mirror overworld/nether/end,OK
https://www.planetminecraft.com/data-pack/mining-dimension-4726054/ OK
https://www.planetminecraft.com/data-pack/the-skylands-dimension crash
https://www.planetminecraft.com/data-pack/heaven-dimension-4787648/ crash

with mod:
https://www.curseforge.com/minecraft/mc-mods/advanced-mining-dimension OK
https://www.curseforge.com/minecraft/mc-mods/immersive-portals-for-forge,with 2 skyland/1 void/2 noise dim OK

All the crash reports are like witch in pastebin

Looking through vanilla code, this should be impossible. No where in vanilla uses immutable maps in the context we add the island info. That said, I dont care to figure out whats going on, so I just added a catch to log the error instead of hard crashing, as there really is nothing I can do if it is immutable.