fix: APP-450 handle undefined errors in buy page when seller is logged in

[r41ph]

Description

https://regennetwork.atlassian.net/browse/APP-450

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• provided a link to the relevant issue or specification
• provided instructions on how to test
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

How to test

1. Login with the testing user
2. Navigate to a project buy page while being logged in with the credits seller account. i.e https://deploy-preview-2536--regen-marketplace.netlify.app/project/C01-010/buy
3. Verify that the error page does no longer appear before being redirected to the projects page

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed all author checklist items have been addressed
• reviewed code correctness and readability
• verified React components follow DRY principles
• reviewed documentation is accurate
• reviewed tests
• manually tested (if applicable)

[netlify]

✅ Deploy Preview for regen-website ready!

Name	Link
🔨 Latest commit	12d1e6a
🔍 Latest deploy log	https://app.netlify.com/sites/regen-website/deploys/672e3f219f850f00086b6797
😎 Deploy Preview	https://deploy-preview-2536--regen-website.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.