Bullet and DAG problem

import direct.directbase.DirectStart
from panda3d.core import Vec3, NodePath
from panda3d.bullet import BulletWorld, BulletDebugNode
from panda3d.bullet import BulletPlaneShape
from panda3d.bullet import BulletRigidBodyNode
from panda3d.bullet import BulletBoxShape

base.cam.setPos(10, -30, 20)
base.cam.lookAt(0, 0, 5)

# World
world = BulletWorld()
world.setGravity(Vec3(0, 0, -9.81))

# Root node A, its content node and neighbour node
node_A = NodePath("region_node_A")
content_A = NodePath("region_content_node_A")
neighbour_A_to_B = NodePath("region_A_to_B")
neighbour_A_to_B.reparentTo(node_A)

# Root node B and its content
node_B = NodePath("region_node_B")
content_B = NodePath("region_content_node_B")
neighbour_B_to_A = NodePath("region_B_to_A")
neighbour_B_to_A.reparentTo(node_B)

# Reparent content_A to node_A and content_B to node_B
node_A.attachNewNode(content_A.node())
node_B.attachNewNode(content_B.node())

neighbour_A_to_B.attachNewNode(content_B.node())
neighbour_B_to_A.attachNewNode(content_A.node())

## A to render!
node_B.reparentTo(render)

neighbour_A_to_B.setPos(-2.5, 0, 0)
neighbour_B_to_A.setPos(-5.0, 0, 0)

# Plane
shape = BulletPlaneShape(Vec3(0, 0, 1), 1)
node = BulletRigidBodyNode('Ground')
node.addShape(shape)
np = render.attachNewNode(node)
np.setPos(0, 0, -2)
world.attachRigidBody(node)

## Debug on
debugNode = BulletDebugNode('Debug')
debugNode.showWireframe(True)
debugNode.showConstraints(True)
debugNode.showBoundingBoxes(False)
debugNode.showNormals(False)
debugNP = render.attachNewNode(debugNode)
debugNP.show()

world.setDebugNode(debugNP.node())

# Boxes
model = loader.loadModel('models/box.egg')
model.setPos(-0.5, -0.5, -0.5)
model.flattenLight()
shape = BulletBoxShape(Vec3(0.5, 0.5, 0.5))
for i in range(10):
    node = BulletRigidBodyNode('Box')
    node.setMass(1.0)
    node.addShape(shape)
    np = None
    if i < 5:
      np = content_A.attachNewNode(node)
    else:
      np = content_B.attachNewNode(node)

    np.setPos(0, 0, 2+i*2)
    world.attachRigidBody(node)
    model.copyTo(np)

# Update
def update(task):
  dt = globalClock.getDt()
  world.doPhysics(dt)
  return task.cont

taskMgr.add(update, 'update')
run()