Fix 'npm start' Error & Site Maintenance

My website works but now throws an 'Unhandled promise rejection' error when launched with 'npm start'. The issue started occurring randomly without any recent changes on my part. I need a professional to fix this issue and maintain the site going forward.

Ideal skills for this job:
- Proficiency in JavaScript and Node.js
- Experience with npm and promise handling
- Familiarity with web development and site maintenance

I have full access to the website's source code, so you will have everything you need to troubleshoot and fix this error.