Component throws unhandled exceptions and crashes after drag over not expanded lazy-loaded node.

It seems that lazy-loading is still an experimental issue, so I have created [PR](https://github.com/frontend-collective/react-sortable-tree/pull/297) to include an example.

Steps to reproduce the actual bug:
1. Go to storybook samples and navigate to **Advanced** -> **Expand with lazy loading of child nodes**.
2. Open Console in web browser to see logs.
2. Try to drag **Egg** node over **Spaceship** node.
3. Unhandled exceptions are logged.
4. If you drag **Egg** node from top direction over center of **Spaceship** node there are even more fatal react exceptions and tree disappears. Please check the recording: 
5. Note: fatal errors occurs and tree disappears only on 16 version of React, on 15 it was working quite stable.

This bug documents the problem referenced in [issue 211](https://github.com/frontend-collective/react-sortable-tree/issues/211).

As for now I do not have any better ideas other than to recreate my old abandoned [PR](https://github.com/frontend-collective/react-sortable-tree/pull/227).

Do you have any ideas how to fix this in most correct way?