[CYS] Fix the failed to load resource error in the CYS whenever the current active theme is not TT4

[albarin]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

Closes #45479

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Ensure the WooCommerce Beta Tester plugin is installed and activated (available on this monorepo).
2. Head over to /wp-admin/tools.php?page=woocommerce-admin-test-helper and enable customize-store feature flag.
3. Click on Tools and run the Reset Customize Your Store and the Delete all products commands.
4. Enable the TT1 theme.
5. Go to wp-admin/admin.php?page=wc-admin&path=%2Fcustomize-store.
6. Open the inspector.
7. Click on Start designing.
8. Make sure you see the following modal:

10. Click on Design a new theme.
11. Check there's no 404 error in the inspector and the flow finishes and it loads the assembler (this is the error this PR is trying to fix, it should not happen anymore).
12. Go to wp-admin/themes.php and make sure the active theme is TT4.

Changelog entry

• Automatically create a changelog entry from the details below.

Significance

• Patch
• Minor
• Major

Type

• Fix - Fixes an existing bug
• Add - Adds functionality
• Update - Update existing functionality
• Dev - Development related task
• Tweak - A minor adjustment to the codebase
• Performance - Address performance issues
• Enhancement - Improvement to existing functionality

Message

CYS - Fix the failed to load resource error in the CYS whenever the current active theme is not TT4

Comment

[github-actions]

Test Results Summary

Commit SHA: 1183042

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 37s
E2E Tests	258	0	0	94	0	352	7m 25s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[github-actions]

Hi @gigitux, @nefeline,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[nefeline]

Excellent work here @albarin, this was a very complex problem with a lot of nuances to it and you did an exceptional work on handling all possible scenarios of failure.

🚀