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Make sure you exported the map, not saved it (if you used gimp)Flatten the image (no layers)Turn alpha channel offMake sure it's in greyscale mode (simply making everything grey isn't enough)And the terrain mask goes into the "terrains" folder as well c:
If your game crashes when you try to get into the map creator, there's an issue with loading the map creator in general. If there's only an issue with the height map or the texture mask the game will crash if you select it from the map creator toolbar, so that isn't the case.There should be a "default" folder inside the "terrains" folder with files such as the ones shown here https://i.gyazo.com/c42aac0f7b32e7ac03f983cd9ac95bbc.png . Are you missing this folder or are any of it's files?
Be sure to check up upon what WolfQueen said above. If you're still having issues, be sure to follow both Height Map and Terrain Map steps thoroughly.
Did you ever move, delete, or modify mapmaker.overlay in the "overlay2" folder, or any other files not relating to your custom map in that matter before this started happening? Did you install any items or textures prior?