Bull reacted well to tip John Oster's pile-driver over just after the hour mark - the closest either side came to breaking the deadlock.
It was the home side who started strongest, Dave Winfield's third-minute header looping over from close range.
Bees captain Edgar Davids fired over twice from outside the area as Barnet tried to find a way into the game, but keeper Graham Stack was the hero for the visitors with a fine save from caretaker boss Gareth Ainsworth on the stroke of half-time.
The second half was a cagey affair until Oster's effort was clawed over by Bull - the Chairboys stopper's only action of an otherwise quiet match proving crucial.
Oster's chance seemed to fire up the home side and they nearly took the lead through Richard Logan 12 minutes from time, the substitute turning his man on the edge of the area but firing agonisingly wide. Wanderers finished the match in the ascendancy but Barnet hung on through a frantic last five minutes to secure the point.