Error, textures disappear after saving? Any ideas?

Hello all! I am having an issue where after i save, the painted terrain disappears. I keep getting this error: " NullReferenceException: Object reference not set to an instance of an object "

and here's a pastebin of the full error message: https://pastebin.com/qsrvVVhs

Also here is an image of the issue: https://imgur.com/WyIYvIM

As you can see, the ground texture is just blank. The green is what i made a while ago and it saved normally, the new mountain i just created WAS green when i was building it, but after saving, the texture just disappeared, and i'm not sure how to fix it.