code: function validateChildrenAreOptionsAndMaximumOnePlaceholder(props, propName, componentName) {
    const validChildren = filterReactChildren(props[propName], (node) => (
      (isOption(node) || isSeparator(node) || isPlaceholder(node))
    ));
    if (React.Children.count(props[propName]) !== React.Children.count(validChildren)) {
      return new Error(`Invalid children supplied to \`${componentName}\`, expected an Option, Separator or Placeholder component from Belle.`);
    }
  
    const placeholders = filterReactChildren(props[propName], (node) => isPlaceholder(node));
    if (React.Children.count(placeholders) > 1) {
      return new Error(`Invalid children supplied to \`${componentName}\`, expected only one Placeholder component.`);
    }
  
    return undefined;
  }
docs: Verifies that the children is an array containing only Options & at maximum
  one Placeholder.