So it sounds like it's no longer crashing anymore right? It's just the heightmap still giving you problems? Is it still crashing because of the heightmap?
If you have already tried resaving over it in Paint and removed the alpha channel and everything, i'm not sure other than to make a dummy heightmap in its place. What I mean by that is just make a brand new heightmap (move the current one into a different folder for safe keeping). The new heightmap doesn't have to have any effort put into it at all. It could be flat even if you want. It's just something that could act as a "refresh" for it. Double check everything is correct: .png format, 513x513, Grayscale, No Alpha Channel and then save it, making sure to use the same name as your previous heightmap name. Then try opening up your map and seeing if the new one appears