After working on an image for a couple of hours, I was interrupted by this OS dialog. Luckily, the app was still responding, so I was able to save the file, and only after that I had to force quit Pixelmator (the Quit item in menu was disabled).
This is the first time I ever see this kind of error (despite using Mac OS for more than 10 years).
I assume that some recent update to Pixelmator introduced a memory leak. I'd be happy to help find and fix it. THanks.