Broken nav after information architecture refactor

We've been struggling with high drop-offs and cognitive load due to our confusing UI navigation. Based on previous feedback, we decided to overhaul our information architecture to simplify user flows and reduce friction.

After implementing the new navigation structure and refactoring the front-end code, the main menu is now completely broken on mobile breakpoints, specifically when trying to open/close the hamburger menu. It works fine on desktop, but mobile users are stuck.

The console is showing a recurring error, which seems to prevent the menu toggle from working correctly:

Uncaught TypeError: Cannot read properties of null (reading 'style')
    at initMobileNav (main.js:123:19)
    at HTMLDocument.<anonymous> (app.js:45:7)

I'm completely stuck trying to debug this and our conversion rates are tanking because mobile users can't navigate. Any ideas on what might be causing this specific JS error or how to approach debugging dynamic mobile navigation issues?

Thanks in advance!