ContactA or ContactB return NULL Pointer Why?

ContactA or ContactB return NULL Pointer Why?
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#1

Sometimes randomely my ContactA or B return NULL pointer. Any ideas why

//WHEN TWO PHYSICS OBJECTS COLLIDE FUCNTION - DANIEL
bool FirstWorld::onContactBegin(cocos2d::PhysicsContact &contact)
{
	PhysicsBody *contactA = contact.getShapeA()->getBody();
	PhysicsBody *contactB = contact.getShapeB()->getBody();

	//CHECK WALL AND VEHICLE COLLISION
	if (contactA->getCollisionBitmask() == 1 && contactB->getCollisionBitmask() == 2)
	{
		if (contactA == playerVehicleObject->getPhysicsBody()) { playerVehicleObject->setSpeed(140); }
		return true;
	}
	else if (contactA->getCollisionBitmask() == 2 && contactB->getCollisionBitmask() == 1)
	{
		if (contactB == playerVehicleObject->getPhysicsBody()) { playerVehicleObject->setSpeed(140); }
		return true;
	}

	if (contactA->getCollisionBitmask() == 2 && contactB->getCollisionBitmask() == 3)
	{
		this->removeChild(contactB->getNode(), true);
		return true;
	}
	else if (contactA->getCollisionBitmask() == 3 && contactB->getCollisionBitmask() == 2)
	{
		cocos2d::Sprite* boom = Sprite::create("TrackOne/WayPoint.png");
		boom->setPosition(contactA->getNode()->getPosition());//HERE SOMETIMES NULL
		this->addChild(boom, 0);
		this->removeChild(contactA->getNode(), true);
		return true;
	}

	return true;
};

Also in this contact function when i use “this->addChild(particles);”, Its seems to add to my vehicle layer. Doesn’t this just add to scene. Why it trails along with my vehicle ?