Nonsense, floofie. We'll get this figured out for ya. <3
At this rate, I would try making a very simple heightmap as a kind of 'tester' (just have a black heightmap with some basic hills, and not much else).
Do the same with a mask (in RBG mode), as well.
Sometimes if a map is too detailed, the game won't want to render it right away. So, test out these basic images. If they work, then try your current heightmap again.
However, if even the testers don't work, I would suggest uninstalling, then reinstalling FH. There could be some corrupt files hanging around that are causing you to crash. The reinstall should fix the issue.
If not, let us know, and we'll try something else. c:
