Thank you for reporting this! You may be able to confirm that the file is in fact saved, although the final field change (that caused the unbalanced braces error) doesn't get included in the save.

The program wouldn't quit unless the file had actually been saved - a failure to save can happen under other circumstances, and will halt the shutdown process. However, in this specific case the failed field change isn't communicated to the save process, so it saves the file without applying the illegal field change, and then quits.