function onlyChild(children) {
    if (!isValidElement(children)) {
      throw new Error('React.Children.only expected to receive a single React element child.');
    }

    return children;
  }
Returns the first child in a collection of children and verifies that there
is only one child in the collection.

See https://reactjs.org/docs/react-api.html#reactchildrenonly

The current implementation of this function assumes that a single child gets
passed without a wrapper, but the purpose of this helper function is to
abstract away the particular structure of children.

@param {?object} children Child collection structure.
@return {ReactElement} The first and only `ReactElement` contained in the
structure.