use React to unmount tooltip, not innerHTML

[brambow]

Thanks so much for this example. I was using it with a style that only has a few layers, which only exist in the US (e.g. there isn't tile coverage for the whole world.):

When my cursor went outside those features I got the following error:

Warning: render(...): It looks like the React-rendered content of this container was removed without using React. This is not supported and will cause errors. Instead, call ReactDOM.unmountComponentAtNode to empty a container.

This isn't something you would encounter using a full OSM-based style, but to account for this I changed the setTooltip function to use ReactDOM.unmountComponentAtNode(this.tooltipContainer); instead of
this.tooltipContainer.innerHTML = ''; for removing the tooltip when there are no features.