We often go to the Bastille Opera and we stay either deep in the 17th or the 13th so both require midnight trips across town on the metro. Paris and the metro are hopping at night. The metro will be full of couples, friends, family groups etc till it closes. We have never had issues at all at night in Paris either walking or on the metro. Our biggest problem with the opera is that it is very full and we often wait for a later train and let the first wave pass.
I don't know where you live, but Paris is not like the US with armed and dangerous people unpredictably out and about at night or areas of the city where you would be nuts to go at night or perhaps even in the day. there is literally nowhere in the city of Paris that I am afraid of at night (we are not out at 2 am -- but often return to ouer apartment at midnight or shortly thereafter.) We have walked from Montmartre around Barbes to an apartment near square Batignolles late at night after visiting friends. On a nice night it is a beautiful walk; on a rainy night we catch the train.
You will be in Central Paris -- I would not give this a thought. (the trains are a bit safer from pickpockets at this hour because they like to go home too -- you must however always be pickpocket proof when on public transport and the Eiffel Tower is a great place to lose your wallet -- so don't carry many valuables and stow them safely out of reach
