function onlyChild(children) {
  !isValidElement(children) ? invariant(false, 'React.Children.only expected to receive a single React element child.') : void 0;
  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#react.children.only

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.