Pagedown editor attach handler to image dialog

Question

I've made my own upload library dialog with Semantic UI's modal. I'm trying to integrate this dialog within the Pagedown editor.

var converter = Markdown.getSanitizingConverter();
var editor = new Markdown.Editor(converter);

editor.hooks.set("insertImageDialog", function(callback) {
    uploadModal.load(); // The dialog HTML gets loaded asynchronously

    $('body').on('selection', '.upload-modal', function(e, src) {
        console.log(src);
        callback(src);
    });

    return true; // tell the editor that we'll take care of getting the image url
});

Everything works fine the first time selecting an image from the dialog, but after that it breaks:

Uncaught TypeError: Cannot call method 'removeChild' of null Markdown.Editor.js

According to this SO question it has something to do with defining the event handler multiple times, but I can't wrap my head around it.

Some more info:

The HTML for initializing the modal is loaded asynchronously, here is the JS code:

var uploadModal = (function() {

    /**
     * Load the upload modal.
     */
    this.load = function() {
        var $modal = $('.upload-modal');

        if ($modal.length) {
            // Show the already existing modal.
            $modal.modal('show');
        } else {
            // Get the HTML
            var request = $.get(appUrl('admin/upload/modal'));

            request.done(function(html) {
                // Make the modal.
                var $modal = $(html);

                $modal.modal({
                    observeChanges: true,
                    onApprove: function () {
                        var upload = $modal.find('.selected.upload'),
                            src = upload.find('.image > img').attr('src');

                        // Bind the event listener which will return the upload's source URL
                        $modal.trigger('selection', src);

                        return true;
                    }
                }).modal('show');

                // Some event handlers here    
            });
        }
    };
})();

I load the modal by calling uploadModal.load() and watch whenever an image is selected by attaching the 'selection' event handler.