You can try linking to iTunes, and syncing a backup, then if you have another device, restoring the backup there to see if the files are saved.
Also, not sure, but maybe turning on iCloud would save the files? Not clear whether Numbers would need to launch in order for the files to get saved to iCloud.
Btw, have you tried completely shutting down the device? Hold the power button until the slider shows, then shut it down. Restart. I've seen some posts where that stops this crashing after an update. Best of luck.