Forge version : 14.23.5.2838
Atum version : 2.0.15
No other mods installed.
Issue was tested in Single Player world. Playing Multiplayer crashes client, gives no info on server.

Issue : When opening a map while any atum.blocks.stone.ceramic.BlockCeramicSlab has a view of the sky, the game will crash to desktop. Blocks affected are highlighted here: ]

Crashlog can be found here : https://hatebin.com/feteebjjna

Steps to duplicate issue : 1) Place down a Ceramic Tile Slab, any color, on the ground in open space. 2) Open a Vanilla Map. 3) Wait a moment for hard crash.

This issue was first discovered with a crash while using Xaero's World Map, while the map was rendering the above blocks. Xaero's World Map was removed and the issue was replicated using a Vanilla Map.
Here is a link to the initial report to Xaero's for reference : https://www.curseforge.com/minecraft/mc-mods/xaeros-world-map/issues/33