Set `_callback` to null when AsyncSelect unmounts.
Set `_callback` to null when AsyncSelect unmounts.

If the AsyncComponent unmounts while the app is fetching the options, the `loadOptions` callback will try to update the component when it eventually resolves.  This will cause React to print a warning message about updating unmounted components.  This pull request sets the `this._callback` field to null when the component unmounts, which tells the component to ignore the results of the callback.
mmpro/react-select,JedWatson/react-select,JedWatson/react-select,submittable/react-select,serkanozer/react-select