Many Ways can be used to do this. *spoilers*
1 - A transparent pole, like acrylic hidden behind the bike.
2 - A normal Pole/broom then edited out.
3 - A Person, standing in shot - holds the bike, Counts from 3, on 2 They let go of the bike, jump back, Shot is taken on 1, on 0 grabs the bike again before it falls over.
4 - Fishing line