Subclassing custom CollectionView Cell doesn't show parent UI

Question

I have a custom cell that i create from xib.

It looks like this:

And i have 2 other cells created only in class that subclass it.

And need to add different views to the containerView

So in my UIViewController i register all cells.

@IBOutlet weak var collectionView: UICollectionView! {
    didSet {
        collectionView.dataSource = self
        collectionView.delegate = self
        collectionView.register(cell1.self, forCellWithReuseIdentifier: "cell1")
        collectionView.register(cell2.self, forCellWithReuseIdentifier: "cell2")
        collectionView.register(UINib(nibName: "ParentCell", bundle: nil), forCellWithReuseIdentifier: "parentCell")
    }
}

and dequeueing the cells.

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

    if indexPath.row == 0 {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell1", for: indexPath) as! cell1
        cell.containerView.backgroundColor = UIColor.yellow
        return cell
    }

    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell2", for: indexPath) as! cell2
    cell.containerView.backgroundColor = UIColor.red

    return cell
}

but the app crashes with

fatal error: unexpectedly found nil while unwrapping an Optional value

when i try to use containerView

Any suggestions how to implement this?
Thanks

---

EDIT:

My problem is that the views from the parent cell are not getting instantiated. I understand what

fatal error: unexpectedly found nil while unwrapping an Optional value

means. i dont understand why are the view not getting instantiated.

The crash is on

cell.containerView.backgroundColor