Mob spawning at y >= 256 produces an error in console

[marcbal]

Please paste the output from /usb version below

14:24:00 Name: uSkyBlock
14:24:00 Version: 2.8.8
14:24:00 Description: Ultimate SkyBlock v2.8.8-bdd3090
14:24:00 Language: fr (fr)
14:24:00 State: d=200, r=80, i=402, p=472, n=true, awe=false
14:24:00 Server: Paper git-Paper-126 (MC: 1.15.2)
14:24:00 State: online=true, bungee=true
14:24:00 ------------------------------
14:24:00 Vault 1.7.3-b131 (ENABLED)
14:24:00 WorldEdit 7.1.0;8e55131 (ENABLED)
14:24:00 WorldGuard 7.0.2-SNAPSHOT;bf2593e (ENABLED)
14:24:00 Multiverse-Core 4.1.0-b775 (ENABLED)
14:24:00 Multiverse-Portals 4.1.0-b805 (ENABLED)
14:24:00 Multiverse-NetherPortals 4.1.0-b763 (ENABLED)
14:24:00 ------------------------------

What steps will reproduce the problem?
I don’t know how to reproduce the bug, but it’s probably because there is some blocks at the highest altitude and mobs try to spawn above them (so at y = 256). Sadly, the error doesn’t show details about the event (location, mob type).

If you have any log-files, please paste them to pastebin.com
https://pastebin.com/uXCLdDJa

(Not following the template will result in closing the issue.)

[Muspah]

Nice catch, thanks very much. It's going through the isPrismarine check, that's a good hint for where to search. I'll check it out when I'm back at the computer.