Fix lesson loading error issue in learning mode for themes that are using referenced styles

[Imran92]

Resolves #7659

8551277-zd-a8c

Proposed Changes

• In theme.json version 3, it's possible to use a reference of a color from another json file instead of explicitly using the name of hexcode of a color. We can use a object with a ref property now. But this function here can only extract the style from a string property, not from a reference object.

So to prevent the error, we've added a check to attempt extracting style only when the value is a string. For proper extraction we've created an issue here #7660 .

Testing Instructions

1. Make sure you are on PHP 8.1 or above
2. Create a course with lessons
3. Ensure learning mode is enabled for that course
4. Download and enable the theme https://wordpress.com/theme/assembler
5. Access the lesson as a student
6. It should render without any issue
7. Now try to do the same from trunk
8. It should throw an error

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[github-actions]

Test the previous changes of this PR with WordPress Playground.

[merkushin]

Thanks, @Imran92!

I reproduced the issue on trunk first, then confirmed the issue is fixed in this PR.

[Imran92]

Two more incidents of this issue have been reported here #7659.