dillon1000/react

Throw for Misue of LocalEventTrapMixin

Summary:
If `getDOMNode()` returns null in `LocalEventTrapMixin`, `listener` will also be null and `accumulateInto` will throw. This changes `LocalEventTrapMixin` to throw a more helpful error message.

Test Plan:
Ran unit test successfully:

```
npm test LocalEventTrapMixin-test.js
```