Bug report: invalid theme gives TypeError: t.querySelector(...) is null

[wisdom-in-snow]

Describe the bug
Cyberchef gave me the following error:

CyberChef encountered an error while loading.

The following browser versions are supported:

Google Chrome 50+
Mozilla Firefox 38+

Your user agent is:
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:136.0) Gecko/20100101 Firefox/136.0

If your browser is supported, please raise an issue including the following details:

TypeError: t.querySelector(...) is null
Filename: https://gchq.github.io/CyberChef/assets/main.js
Line: 2
Column: 8177840
Error: TypeError: t.querySelector(...) is null
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:136.0) Gecko/20100101 Firefox/136.0
CyberChef version: 10.19.4

To Reproduce

1. visit: https://gchq.github.io/CyberChef?theme=foo where foo is not a valid theme name.

It does not occur when foo is a valid theme name or for https://gchq.github.io/CyberChef

Expected behaviour

Cyberchef should probably give an "invalid theme" message or load with the default theme.

Desktop (if relevant, please complete the following information):

• OS: Ubuntu 24.04
• Browser: firefox 136 and chrome 133.0.6943.126 (Official Build) (64-bit)
• CyberChef version: 10.19.4