Your Heightmap might be affecting the map and crashing the game when you go to edit it. Make sure that your heightmap is:
-Grayscale
-513x513
-One layer, alpha channel off
-Flattened
If this is not the case, then check your Terrain Mask. You can only use 3 colors on this image, that being the extremes of Red, Green, and Blue. Make sure each color is at its extreme, for example, Red: (R:255, G:0, B:0). This image should also be 1 layer, flattened and merged.