attachPhysicalNode error

 # Physics Setup
        self.physicsNode = NodePath(PandaNode("PhysicsNode"))
        self.actorNode = ActorNode("momentum")
        self.actorNodePhysics = self.physicsNode.attachNewNode(self.actorNode)
        self.actorNode.getPhysicsObject().setMass(69) #about 3 lbs
        self.pusherForce = LinearVectorForce(10,10,20)

This code worked with v1.5.2. Now I am using v1.8.1, I don’t understand why I’m getting this error :
AttributeError: ‘NoneType’ object has no attribute ‘attachPhysicalNode’
Has there been a change which I am missing?

You need to call base.enableParticles() in order for base.physicsMgr to become initialized.

Thank you.