Console error on documentation pages.

When loading the documentation pages in the dashboard. A console error is displayed.
```
Uncaught TypeError: t[(t.length - 1)] is undefined
u highlightjs-line-numbers.min.js:97
d highlightjs-line-numbers.min.js:93
s highlightjs-line-numbers.min.js:76
setTimeout handler*g highlightjs-line-numbers.min.js:145
s highlightjs-line-numbers.min.js:75
l highlightjs-line-numbers.min.js:68
i highlightjs-line-numbers.min.js:61
i highlightjs-line-numbers.min.js:60
script.js:2
2 highlightjs-line-numbers.min.js:97:16
```

It's worth noting the script in question is quite old (version v9.15.6) and there are [newer versions](https://highlightjs.org/news). [Releases list](https://github.com/highlightjs/highlight.js/releases).

The syntax highlighting still seems to work fine.

Steps to reproduce:
1. Load a documentation page in the dashboard
2. Check the JS console in the browser.

The page should show correctly and there should be no errors in the console.


Acceptance criteria:

- [ ] The page should show correctly without errors in the console