JS function breaks – ancestor is null

Trying to troubleshoot why this function is failing and my JS is very very rusty. It’s intended to collapse/expand sections on the page and works when expanding, but then breaks when it’s clicked again and is supposed to collapse that section.

Here’s the calling function (screenshot):

Here’s the calling function (full code):

NS.run_expansion = function(categorynode) {
    var categorychildren = categorynode.one(SELECTORS.CONTENTNODE),
        self = this,
        ancestor = categorynode.ancestor(SELECTORS.COURSECATEGORYTREE);

    // Add our animation to the categorychildren.
    this.add_animation(categorychildren);


    // If we already have the class, remove it before showing otherwise we perform the
    // animation whilst the node is hidden.
    if (categorynode.hasClass(CSS.SECTIONCOLLAPSED)) {
        // To avoid a jump effect, we need to set the height of the children to 0 here before removing the SECTIONCOLLAPSED class.
        categorychildren.setStyle('height', '0');
        categorynode.removeClass(CSS.SECTIONCOLLAPSED);
        categorynode.setAttribute('aria-expanded', 'true');
        categorychildren.fx.set('reverse', false);
    } else {
        categorychildren.fx.set('reverse', true);
        categorychildren.fx.once('end', function(e, categorynode) {
            categorynode.addClass(CSS.SECTIONCOLLAPSED);
            categorynode.setAttribute('aria-expanded', 'false');
        }, this, categorynode);
    }

    categorychildren.fx.once('end', function(e, categorychildren) {
        // Remove the styles that the animation has set.
        categorychildren.setStyles({
            height: '',
            opacity: ''
        });

        // To avoid memory gobbling, remove the animation. It will be added back if called again.
        this.destroy();
        self.update_collapsible_actions(ancestor);
    }, categorychildren.fx, categorychildren);

    // Now that everything has been set up, run the animation.
    categorychildren.fx.run();
};

And then fails here (screenshot):

Failing function (full code):

NS.update_collapsible_actions = function(ancestor) {
    var foundmaximisedchildren = false,
        // Grab the anchor for the collapseexpand all link.
        **togglelink = ancestor.one(SELECTORS.COLLAPSEEXPAND);**

    if (!togglelink) {
        // We should always have a togglelink but ensure.
        return;
    }

    // Search for any visibly expanded children.
    ancestor.all(SELECTORS.CATEGORYWITHMAXIMISEDLOADEDCHILDREN).each(function(n) {
        // If we can find any collapsed ancestors, skip.
        if (n.ancestor(SELECTORS.CATEGORYWITHCOLLAPSEDLOADEDCHILDREN)) {
            return false;
        }
        foundmaximisedchildren = true;
        return true;
    });

    if (foundmaximisedchildren) {
        // At least one maximised child found. Show the collapseall.
        togglelink.setHTML(M.util.get_string('collapseall', 'moodle'))
            .addClass(CSS.COLLAPSEALL)
            .removeClass(CSS.DISABLED);
    } else {
        // No maximised children found but there are collapsed children. Show the expandall.
        togglelink.setHTML(M.util.get_string('expandall', 'moodle'))
            .removeClass(CSS.COLLAPSEALL)
            .removeClass(CSS.DISABLED);
    }
};

I have Googled this and played around with the code a bit, but I’m not making any headway. As I said, my JS is very rusty, so please be kind 🙂