It depends on where you would like to go. Paris Gare du Nord is quite safe. There are a lot of people there and even late at night I never had to worry about safety, BUT, do not attract attention with a big camera or any other stuff that can make you look like a tourist.
It is set in and around Les Halles, the enormous, busy central market of 19th-century Paris. Les Halles, rebuilt in cast iron and glass during the Second Empire was a landmark of modernity in the city, the wholesale and retail center of a thriving food industry.