I successfully added some custom JS and CSS to my head section ([look here][1]).

But If I try to add a script at the block `before_body_end` then I get:


>     There has been an error processing your request
>
>     Invalid method Mage_Core_Block_Text_List::addItem(Array
>     (
>         [0] => skin_js
>         [1] => black/NewsletterPopup/js/main.js
>     )


**app\design\frontend\base\default\layout\black\newsletter_popup.xml**

    <?xml version="1.0"?>
    <layout version="0.1.0">
        <default>
            <reference name="head">
                <action method="addItem">
                    <type>skin_js</type>
                    <script>black/NewsletterPopup/js/jquery.magnific-popup.min.js</script>
                </action>
                <action method="addCss">
                    <stylesheet>black/NewsletterPopup/css/magnific-popup.css</stylesheet>
                </action>
                <action method="addCss">
                    <stylesheet>black/NewsletterPopup/css/main.css</stylesheet>
                </action>
            </reference>
    
            <reference name="before_body_end">
                <action method="addItem">
                    <type>skin_js</type>
                    <script>black/NewsletterPopup/js/main.js</script>
                </action>
            </reference>
        </default>
    <!-- ... -->

  [1]: https://magento.stackexchange.com/questions/228531/magento-1-x-custom-js-css-not-found