Custom SKNode object is not shown | Swift

Question

I programmed an SKButton class. The problem is that the button doesn't show up in my scene even though I added it to this one. I also programmed a joystick, but it is displayed. What did I do wrong?

SKButton:

import SpriteKit

class SKButton: SKNode {
    let background: SKSpriteNode = SKSpriteNode(color: UIColor.red, size: CGSize(width: 100, height: 10))
    let text: SKLabelNode = SKLabelNode()
    
    var buttonText: String = ""
    var backgroundColor: UIColor = UIColor.red
    var fontSize: CGFloat = 0.0
    var fontName: String = ""
    var fontColor: UIColor = UIColor.white
    var buttonAction: (() -> ())?
    
    init(text: String, backgroundColor: UIColor, fontName: String, fontSize: CGFloat, fontColor: UIColor) {
        super.init()
        
        buttonText = text
        self.backgroundColor = backgroundColor
        self.fontName = fontName
        self.fontSize = fontSize
        self.fontColor = fontColor
        
        initialize()
        
        addChild(background)
        addChild(self.text)
        
        print("init")
    }
    
    func tapped(touch: UITouch) {
        let node: SKNode = atPoint(touch.location(in: self))
        
        if let name = node.name {
            if name == "B" || name == "T" {
                if let buttonAction = buttonAction {
                    buttonAction()
                    print("tapped")
                }
            }
        }
    }
    
    func initialize() {
        text.verticalAlignmentMode = SKLabelVerticalAlignmentMode.center
        text.horizontalAlignmentMode = SKLabelHorizontalAlignmentMode.center
        
        text.fontSize = fontSize
        text.fontColor = fontColor
        text.fontName = fontName
        text.text? = buttonText
        text.position = CGPoint.zero
        text.zPosition = 1
        text.name = "T"
        
        background.zPosition = 0
        background.position = CGPoint.zero
        background.size = CGSize(width: text.frame.width * 1.25, height: text.frame.height * 1.5)
        background.color = backgroundColor
        background.name = "B"
        print("initialize")
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

I checked YouTube and StackOverflow, but couldn't find an answer. If there is, I have overlooked it. But I've really tried hard to find an answer. That is why I am asking the question here.

I created a constant of my button in the scene: let startButton: SKButton = SKButton(text: "START", backgroundColor: UIColor.red, fontName: "Arial", fontSize: 20, fontColor: UIColor.white)

Then I moved the button to the right place: startButton.position = CGPoint(x: size.width / 2, y: size.height / 2 + startButton.background.size.height * 0.75)

And then I added it to my scene: addChild(startButton)