When you get an object at a particular index from NSArray it always returns and object of type id
which means that it does not include any class related information in it.
Try the following code
id object = _arrayOfChunks[0]
if([object isKindOfClass [SKNode class]])
{
[(SKNode *)object setPosition:CGPointMake(screenSizeHalfW, screenSizeHalfH-100)];
}
This should silence the error.
We do a check to make sure that the object accessed is actually of type SKNode just make sure we do not get an exception if in future objects of other type are added to the array.