Sprite Kit Collision error - Swift - ios

Question

I am new to IOS dev and am currently having some issues with sprite kit collisions in the didBeginContact method.
How do I break out of, or stop didBeginContact from running if one of the colliding physics bodies are removed.
eg:
1 bullet collides with 2 overlapping enemies. Because the bullet hits enemy one and is destroyed, the collision check running on the second enemy throws an exception because the bullet no longer exists.
I have tried checking for nil and NSNULL values with no luck.
The error code that I receive is "Thread 1: EXC_BAD_INSTRUCTION(code=EXC_I386_INVOIP,subcode=0x0)" and occurs when trying to check the category bit mask (Because the torpedo no longer exists).
CODE:
var bodyA:SKPhysicsBody = contact.bodyA
var bodyB:SKPhysicsBody = contact.bodyB
if(contact.bodyA.categoryBitMask == alienCategory && contact.bodyB.categoryBitMask == alienCategory){
return
}
if((bodyA.categoryBitMask == alienCategory) && (bodyB.categoryBitMask == photonTorpedoCategory)){
torpedoDidCollideWithAlien(bodyA.node as SKSpriteNode)
}
var currTorpedo:SKSpriteNode = contact.bodyB.node as SKSpriteNode
var currAlien:SKSpriteNode = contact.bodyA.node as SKSpriteNode
currTorpedo.removeFromParent()
currAlien.removeFromParent()
}
func torpedoDidCollideWithAlien(alien:SKSpriteNode){
aliensDestroyed++
label.text = ("Current Score: " + String(aliensDestroyed))
}

Answer 1

Perhaps you could delay removing the torpedo from the scene until the next scene 'update'. You could flag the torpedo as disabled/inactive so it doesn't impact your 2nd overlapping enemy. Something like this:
class YourScene: SkScene {
// Array of disabled torpedos
var disabledTorpedos = SKSpriteNode()[]
func didBeginContact(contact: SKPhysicsContact) {
var bodyA:SKPhysicsBody = contact.bodyA
var bodyB:SKPhysicsBody = contact.bodyB
if(contact.bodyA.categoryBitMask == alienCategory && contact.bodyB.categoryBitMask == alienCategory){
return
}
if((bodyA.categoryBitMask == alienCategory) && (bodyB.categoryBitMask == photonTorpedoCategory)){
torpedoDidCollideWithAlien(bodyA.node as SKSpriteNode)
}
var currTorpedo:SKSpriteNode = contact.bodyB.node as SKSpriteNode
var currAlien:SKSpriteNode = contact.bodyA.node as SKSpriteNode
// Created a new photonTorpedoDisabledCategory so the torpedo cannot hit a 2nd alien
currTorpedo.physicsBody.categoryBitMask = photonTorpedoDisabledCategory
disabledTorpedoes.append(currTorpedo)
// Hide torpedo from scene
currTorpedo.hidden = true
currAlien.removeFromParent()
}
override func update(currentTime: NSTimeInterval) {
// loop through disabledTorpedos array and removeFromParent
}
}