register_lightbox gets the lightbox element only when it's activated

It looks like `register_lightbox` happens before `NumbasExamElement.init`, which sets `this.lightbox`.

A way round this is to only get `this.lightbox` in the function that activates the lightbox, since that's where it's used.

Author: christianp

themes/default/files/scripts/display-base.js

@@ -465,7 +465,6 @@ class NumbasExamElement extends HTMLElement {
      */
     register_lightbox(element) {
         const register_image = (img) => {
-            const {lightbox} = this;
 
             var wrapper = document.createElement('span');
             wrapper.setAttribute('class', 'lightbox-image-wrapper');
@@ -492,6 +491,7 @@ class NumbasExamElement extends HTMLElement {
             button.title = button.ariaLabel = R('lightbox.zoom in on image');
 
             const activate = () => {
+                const {lightbox} = this;
                 lightbox.innerHTML = '';
                 var elem = img.cloneNode();
                 const scope = display_util.find_jme_scope(img);