Your phone can catch on fire while charging overnight, although the chances of your phone catching on fire when charging overnight are very low.
As long as your phone and phone charger is in good shape and the cord is not frayed then the phone should be very safe to charge overnight or at any other time.
Your phone has a built in battery management system that will shut off the charging once your phones battery is charged.
So your phones battery won't overcharge no matter how long you leave it plugged in.
When I'm not using my phone I always have it plugged in and on the charger either in my car or at home and even at night.
Never had an issue with the phone charging overnight or any other time.
Just make sure your phone charger cord is in good shape and your phone should not catch on fire.