It's probably out of video ram. The first thing you can do is look in the task manager for any previews that didn't close correctly and end them. They usually are called temp.exe, temp2.exe, ...
The next thing to consider doing is use smaller texture sizes. You can also keep the number of open layouts in the editor to a minimum. Keep in mind that both the editor and preview use vram so when making the game you'll be using roughly a little under 2x the amount of vram your game will use when exported.