Hmm...
When you say 'testing' your map, do you mean testing it while you're in map maker? Or do you mean logging into FeralHeart and testing it with a character you've created?
If it's a particularly detailed map, it might be lag that's causing you to crash (just a guess, though). You could try decreasing the workload on your computer by taking any extra, unused maps (portals in Cape) out of Exports and saving them elsewhere for later.
It depends on a few different factors. I hope we can get it worked out for you soon!