If I'm on the tops when reaching for the bottle/food/pocket/signalling, I keep my bar hand on the tops. Ditto for hoods, and drops.
If I have to brake suddenly while one-handed, part of my reflex is usually to bring the other hand back to the bar, even if there's a bottle or banana in it at the time. I don't grip the bar, just press the knuckle or rest my wrist against it.
Maybe it's worth your while to practice braking one-handed.