error removing child node from parent node in spriteKit Swift4 - xcode

Question

I made a parallax background, in GameScene.sks I added an empty node, added two sprites to an empty node. I have a reset button, when I click on it I need the background to be removed and added to its position (restarted), but when I add in scrollBg.removeAllChildren restart function I error occurs, how do I properly add and remove children from the scene ?
import SpriteKit
import GameplayKit
class GameScene: SKScene {
var player: SKSpriteNode!
var scrollBg: SKNode!
var spawnTimer: CFTimeInterval = 0
let fixedDelta: CFTimeInterval = 1.0/60.0 /* 60 FPS */
let scrollSpeed: CGFloat = 700
var sinceTouch: CFTimeInterval = 0
func resetGameScene() {
scrollBG.removeAllChildren()
player.removeAllChildren()
player.position = CGPoint(x: 590 , y: 690)
pauseButton()
}
override func didMove(to view: SKView) {
physicsWorld.contactDelegate = self
player = childNode(withName: "player") as? SKSpriteNode
scrollBg = childNode(withName: "scrollBG")!
resetGameScene()
}
func scrollWorld() {
scrollBg.position.y -= scrollSpeed * CGFloat(fixedDelta)
for ground in scrollBg.children as! [SKSpriteNode] {
let groundPosition = scrollBg.convert(ground.position, to:
self)
if groundPosition.y <= -ground.size.width {
let newPosition = CGPoint(x: groundPosition.x, y:
(self.size.width ) + ground.size.width * 2)
ground.position = self.convert(newPosition, to:
scrollBg)
}
}
}
override func update(_ currentTime: TimeInterval) {
sinceTouch+=fixedDelta
spawnTimer+=fixedDelta
scrollWorld()
}
}