CSSTransitionGroup throws when a child returns null from render #2619
Description
I came across this, when using react-router. When loading a route initially where the route handler is actually inside a ReactCSSTransitionGroup and then removing it from the transition group leads to an Uncaught TypeError: Cannot read property 'addEventListener' of null. The weird thing is that it only happens on initial load of a nested route, not when the same view is being pulled up via the menu. I'm not sure if this is an issue related react-router or if it belongs to react. But the thing is that the RouteHandler component either returns a react element, or it returns null. It seems like an edge case maybe, because it only happens in this specific initial render setup.
It could be fixed, by adding a guard to the transition() method of ReactCSSTransitionGroupChild:
if (!node) {
finishCallback && finishCallback();
return;
}