If you’re getting a black CBM, it’s because your CBM wasn’t converted to 256 color, it was RGB. You probably remembered that step after you rebuilt it with the grid lines checked, which is the real reason it worked.
I would never tell you not to save between each add, but unless you change modes (the road tab etc) you can add as much work as you are willing to risk between saves. Just be sure to save and reopen the terrain BEFORE AND AFTER you change modes OR try to build it. In the previous version(s), some of the necessary files weren’t built unless you saved, and it crashed the TE when it went to use them.