We're dealing with pretty regular bedwetting right now with our four-year-old sons, who we switched to underwear all at once (daytime and nighttime together) around a year and a half ago. Our pediatrician assured us that the best thing is to just let them wet the bed, and that pull-ups at night actually delay the process of them learning to get up to use the bathroom at night. The same goes for waking them up in the middle of the night for a "bathroom break" (something we considered) and anything else that might act as a crutch to simply learning to recognize their own body cues and to deal with them appropriately on their own.
So I'd say that if your child is already out of diapers completely during the day, then your best bet is to just stop using pull-ups at night at the same time. You may end up dealing with a lot of late-night changes of pajamas and/or bed sheets, but your child will likely learn fastest that way.
Another thing to try that can help your child avoid wetting the bed (without interfering with their learning to recognize body cues) is to limit the amount of liquid near bedtime -- though do be sure they're drinking enough during the day, since adequate liquid intake is still very important.
We have a lot of talks with our boys about bedwetting (just to gently remind them that they should try to wake up to go in the middle of the night, and to explain why they can't have one last drink of water immediately before going to sleep -- NOT to shame them or blame them for wetting the bed) and the biggest obstacle we're facing currently is convincing them that when they pee in their sleep, that they really are peeing in real life. They're convinced that somehow their beds are just getting wet on their own, since (as they put it) when they pee in their sleep "that's just pretend pee, daddy."