One thing I noticed in your file is that you load three heightfields, of which two aren't used. That still burdens TG (depends on size of course), so delete anything you don't use. With low RAM (apparently), your memory easily clogs, and once you hit your RAM ceiling, TG crashes.
Also with populations; try to use trees that are not too heavy in MB's, and place them sparingly and efficiently (e.g. no trees behind the camera). For far trees, there are ways to do it procedurally (search for 'fake trees').
If you don't really need the compute terrain, disable it, and TG will render faster. I know it's a bit against the convention, but it often works good enough, especially in scenes like yours.
But if you really want to dive into TG, aim for a better machine or at least more memory.