Map editor

[Ken_Shadow]

I've been trying to make my own city but in order to do so I need a good bit of flat ground, So I copied the navezgane world and put it in a folder named "My World". I went into World editor and loaded it. However when clearing or leveling the ground enough it tends to undo what I've cleared or leveled. I saw a video on youtube where a guy edited the splat3.png and other files using paint.net to make the world as you see fit but opening the files in paint.net I see a blank image. If I try to make a New world in the editor I get the error "Nullreferenceexception object reference not set to an instance of an object". I've tried Uninstalling and reinstalling the game and verify integrity of game files and still having the same problem. How do I make a big section of the map flat without it undoing itself and how do I fix this Null error?