is it safe to travel from Gare du Nord train station to les Halle's @ 10:00 in the night. The 2nd ARR
10pm is hardly late at night! There will still be loads of people around, as well as transport staff.
The only things you need to be concerned about (regardless of time of day) are pickpocketing and opportunistic theft, exactly the same as in the vast majority of cities and crowded places worldwide. Use city commonsense, which applies everywhere:
1. Keep cash, cards, passport and important valuables (including your phone) safely *underneath* your clothing and never, ever in any outside pocket (including front trouser pockets), backpack/daypack or handbag/purse. Pickpockets across the world are exceptionally skillful; never underestimate their abilities.
2. Don't get drawn into conversation with hawkers, beggars, people who offer to 'help' you etc. Distraction is a worldwide and commonplace pickpocketing ploy. Just ignore them and keep walking.
3. Always wear your bag with the strap across your body and, if it's very crowded, hold it in your arms.
4. Don't let anyone 'help' you with your bags. Just say 'Non, merci' and keep walking. Stay polite but be firm.
5. Stay alert and aware of what is going on around you, most especially in crowded situations. Railway stations (especially those with links to airports) are favourite targets for pickpockets because they know arriving passengers are often tired, disoriented and much less alert.
Follow those commonsense rules and you need be no more concerned about using public transport at 10pm in Paris than you would be using public transport anywhere else.
Enjoy your trip! :-)