fatal error: unexpectedly found nil while unwrapping an Optional value for physics body contact - ios

Question

I can't seem to figure out which optional value it is talking about or why i am getting this error. I checked my score integer and made sure that I declared its value is 0 until it makes contact with the enemy. In the simulator the counter counts the first 4 or 5 enemies then crashes.
var score = Int? ()
var scoreLabel = UILabel ()
override func didMoveToView(view: SKView) {
scoreLabel.text = "\(score)"
scoreLabel = UILabel(frame: CGRect(x: 0, y: 0, width: 100, height:
20))
scoreLabel.textColor = UIColor.blackColor()
score = nil
if score == nil {
score = 0
scoreLabel.text = "\(score!)"
}
func didBeginContact(contact: SKPhysicsContact) {
let firstBody : SKPhysicsBody = contact.bodyA
let secondBody : SKPhysicsBody = contact.bodyB
if ((firstBody.categoryBitMask == PhysicsCategory.bullet) &&
(secondBody.categoryBitMask == PhysicsCategory.enemy) ||
(firstBody.categoryBitMask == PhysicsCategory.enemy) &&
(secondBody.categoryBitMask == PhysicsCategory.bullet)) {
//i get the error next line
collisionWithBullet((firstBody.node as! SKSpriteNode),
bullet: (secondBody.node as! SKSpriteNode))
}
}
func collisionWithBullet(enemy: SKSpriteNode, bullet: SKSpriteNode){
score? += 1
scoreLabel.text = "\(score!)"
enemy.removeFromParent ()
bullet.removeFromParent ()
}

Answer 1

Change score to
var score = 0
instead of
var score = Int? ()
and instead of this part
score = nil
if score == nil {
score = 0
scoreLabel.text = "\(score!)"
}
write only this
scoreLabel.text = "\(score)"
Edit:
instead of this part
collisionWithBullet((firstBody.node as! SKSpriteNode),
bullet: (secondBody.node as! SKSpriteNode))
do something like this
if let firstNode = firstBody.node as? SKSpriteNode,
secondNode = secondBody .node as? SKSpriteNode {
collisionWithBullet((firstNode),
bullet: (secondNode))
}