Check children is not null

[kengreeff]

Javascript believes that null is an object. If children is null, the evaluation of typeof this.props.children === 'object' returns true`.

When evaluating the next line, typeof child.type === 'string' the script crashes because it can't check type of null.

[InsOpDe]

@noelzubin is there anything holding that PR back? Because the current master breaks things