title: Add custom cell in childViewController
body: <p>I have a <code>childViewController</code> on a <code>UICollectionViewController</code>. I have so my childViewController appears on the screen. But when I register a cell <code>collectionView?.register(MyCustomCell.self, forCellWithReuseIdentifier: CellId)</code> and use <code>numberOfItemsInSection</code> and <code>cellForItemAt</code>. The problem is that the cells wont appear, I have checked the code so it´s right. Can it be something with the childViewController? </p>