We visited there last week, and were standing in the large square at the front of the Cathedral taking photos (difficult, with the sun behind the building) when a lady walking into the Mairie warned us to be very careful of thieves, who would snatch our cameras and run away. We have read warnings that this is a problem in the area, but were surprised to have someone French (and presumably local) say it to us.
The front of the churchOf course, the warning made us look around for a potential thief, but the square appeared to be almost deserted except for other tourists taking photos, and it wasn't that busy when we left after visiting the tombs, either.
One of the portalsStill - you have been warned. The Basilica is worth visiting, but keep your street smarts happening at all times.