Why i can not use controller.ai.walker to control walkers in carla?

[CaoxyStar]

I try to get a walker which is controlled by ai in carla, i refered to the official tutorial and use the below code, but the walker just stayed static.
`
walker_bp = random.choice(world.get_blueprint_library().filter('walker.pedestrian*'))
spawn_point = carla.Transform()
spawn_point.location = world.get_random_location_from_navigation()
ego_walker = world.try_spawn_actor(walker_bp, spawn_point)

walker_controller_bp = world.get_blueprint_library().find('controller.ai.walker')
ai = world.spawn_actor(walker_controller_bp, carla.Transform(), ego_walker)

ai.start()
ai.set_max_speed(1 + random.random())
target = world.get_random_location_from_navigation()
ai.go_to_location(target)
`