App crashes when trying to add SKNode from if statement - ios

Question

I'm trying to create a moving background on top of a still background using a NSTimer, it works fine until I call the command to add another child. Here is the error I'm getting.
Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attemped to add a SKNode which already has a parent: <SKSpriteNode> name:'(null)' texture:[<SKTexture> 'groundGrass.png' (53 x 71)] position:{159, 35} size:{808, 71}
Here is my code to move the ground:
-(void)moveGroundGrass{
groundGrass.position = CGPointMake(groundGrass.position.x -1, groundGrass.position.y);
if (groundGrass.position.x < 160){
[self addChild:groundGrass];
}
Here is the rest:
-(id)initWithSize:(CGSize)size {
if (self = [super initWithSize:size]) {
/* Setup your scene here */
SKSpriteNode *background = [SKSpriteNode spriteNodeWithImageNamed:#"background"];
background.position = CGPointMake(CGRectGetMidX(self.frame), CGRectGetMidY(self.frame));
background.xScale = 0.9;
background.yScale = 1.1837f;
[self addChild:background];
groundGrass = [SKSpriteNode spriteNodeWithImageNamed:#"groundGrass"];
groundGrass.position = CGPointMake (404, 35);
[self addChild:groundGrass];
moveGround = [NSTimer scheduledTimerWithTimeInterval:0.01f target:self selector:#selector(moveGroundGrass) userInfo:nil repeats:YES];
}
return self;
}
When groundGrass.png moves past the point on screen where it is supposed to add another child node, it crashes...

Answer 1

You arent creating a new groundGrass in the moveGroundGrass method. You need to create a new one like you did in the initWithSize:
In an Obj-C you create an object like this: object *name [object new]; where object can be f.e. UIView.
If you want to have easy access to the objects you are creating you can add them into an NSMutableArray but if you only want to add them and then do nothing to them(not recommended) you can just create it and then add it to the view.