I clicked to add a new terrain, like I did in Divinity Engine 1 editor and set the width and height of the new terrain to 256 x 256. It asked me to select a terrain material, so I selected "TR_Base_RockSlate_A_Grass_A_PBR" and it went "Not Responding" and then just stayed like that.

Did I do something wrong?

GMAddon1: Adds almost every single monster and NPC from the main campaign into GM mode, plus a few large custom maps to use in GM mode.