Forge 13.20.0.2315
Futurepack 25.2.76

I wish I had more than a crash report here, but this happened when I was logged off. It may be worth noting I have done nothing involving Futurepack aside from raiding structures yet on this world. This was just a couple seconds after I had logged off, and the game crashed with a CME traced down to your WorldTickEvent handler FPMain.java:530.
Full crash report: https://gist.github.com/Shadows-of-Fire/8796ad46a1dba93cc257f78b9b05e77f

Since this occured during saving, it also caused this crash report on next startup, but this was a one-time occurrence.
Full crash report: https://gist.github.com/Shadows-of-Fire/1e6c508b6f6fc8dff4d76a1160a2b880

Finally, the entire fml-server log of the first report.
https://gist.github.com/Shadows-of-Fire/c8e70d18d73020a86c35ff38cb540a56
Be warned, that is a 250k line log file.

This is interesting, because i am just calling a normal minecraft method to get some entittys...

Has this only happened once or more often ?

The bad thing is, this is cause by someone modifieng the entity list while saving most likely from another thread, but the crash is cause by my mod.

You can try this version It should not crash again, but this crash was very odd, and seemd very random.

I have yet to see it again. Will reopen if I csn reproduce.