Broken animation finishing after unmounting

[xStrom]

If ReactTransitionGroup is unmounted before all its animations have finished a javascript error is thrown complaining about reading properties of undefined.

Specifically the callbacks _handleDoneAppearing, _handleDoneEntering, _handleDoneLeaving have:

let component = this.childRefs[key];
// The following line (and its analogues) will throw with:
// Uncaught TypeError: Cannot read property 'componentDidLeave' of undefined
if (component.componentDidLeave) {

This bug is especially common with long running animations, however it can also happen with short animations if the timing is right.