Illustrator quits when I try to close a file that is still saving

Hi there,

Sorry to hear about your experience. Would you mind sharing some more information like:
1. Version of OS and Illustrator.
2. Is it happening with a specific file or with all the files?
3. Do you save files locally or on a network?

All this information will help us understand the issue and work accordingly.

You may also try turning off the Save in Background option under Preferences > File Handling & Clipboard.

Let us know how it goes.

Regards,
Ashutosh