Why does ItemContainerGenerator return null?

Question

I have a ListBox, and I need to set its ControlTemplate to a Virtualizing WrapPanel which is a class that extends VirtualizingPanel, using a style that looks like this:

Answer 1

I think I had a similar problem and this helped:

var necessaryChidrenTouch = this.Children;
IItemContainerGenerator generator = this.ItemContainerGenerator;

... for some reason you have to "touch" the children collection in order for the ItemContainerGenerator to initialize properly.