Fix typo collision detection in stack()
Fix typo collision detection in stack()

Avoid comparing items that have not been positioned yet (item.dimensions.top is null)
mkawczynski07/react-calendar-timeline,mkawczynski07/react-calendar-timeline,namespace-ee/react-calendar-timeline,signalwerk/react-calendar-timeline,mkawczynski07/react-calendar-timeline,namespace-ee/react-calendar-timeline,signalwerk/react-calendar-timeline