Is most probably because the node (first parameter) that you want to insert in DOM is null or undefined. You can reproduce this error by running:

document.body.insertBefore(null, document.body.firstChild); 

You must check the line that returns the element reference to draggable :

document.querySelector('.dragging') // must be not null, you can run in console and/or debug using the devtools to see what is happening

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