TabBar with moving selectionIndicatorImage

TabBarController.swift

import UIKit

class TabBarController: UITabBarController {

  var indicatorImage: UIImageView?
    
  override func viewDidLoad() {
    super.viewDidLoad()
        
    let numberOfItems = CGFloat(tabBar.items!.count)
    let tabBarItemSize = CGSize(width: (tabBar.frame.width / numberOfItems) - 20, height: tabBar.frame.height)
    indicatorImage = UIImageView(image: createSelectionIndicator(color: UIColor(red:0.18, green:0.66, blue:0.24, alpha:1.0), size: tabBarItemSize, lineHeight: 4))
    indicatorImage?.center.x =  tabBar.frame.width/4/2
    tabBar.addSubview(indicatorImage!)
  }
    
  override func tabBar(_ tabBar: UITabBar, didSelect item: UITabBarItem) {
    UIView.animate(withDuration: 0.3) {
      let number = -(tabBar.items?.index(of: item)?.distance(to: 0))! + 1
      if number == 1 {
        self.indicatorImage?.center.x =  tabBar.frame.width/4/2
      } else if number == 2 {
        self.indicatorImage?.center.x =  tabBar.frame.width/4/2 + tabBar.frame.width/4
      } else if number == 3 {
        self.indicatorImage?.center.x =  tabBar.frame.width/4/2 + tabBar.frame.width/2
      } else {
        self.indicatorImage?.center.x = tabBar.frame.width - tabBar.frame.width/4/2
      }
    }
  }

  func createSelectionIndicator(color: UIColor, size: CGSize, lineHeight: CGFloat) -> UIImage {
    let rect: CGRect = CGRect(x: 0, y: size.height - lineHeight, width: size.width, height: lineHeight )
    UIGraphicsBeginImageContextWithOptions(size, false, 0)
    color.setFill()
    UIRectFill(rect)
    let image: UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    UIGraphicsEndImageContext()
    return image
  }
}

How can I implement that TabBar? It always crashes as the TabBar has no Items when it first initializes... Would love to see an example!