Resolve console error when using "Sort by" or "Order by" on Patterns page

[devansh016]

What?

This PR resolves a console error that occurs when interacting with the "Sort by" or "Order by" dropdowns on the Patterns page. A null check for contentDocument has been added to ensure iframe initialization happens only after the iframe content is fully loaded.

Why?

The error prevents the proper functioning of the Patterns page, impacting the user experience. This issue was reproducible across multiple themes and environments, as detailed in the related issue.

Related Issue: #68562

How?

The fix introduces a null check for contentDocument in the iframe initialization logic. This ensures that operations on the iframe are only executed when its content is available, preventing the TypeError that previously occurred.

Introduced in #38855 ( since we no longer try to initialise the iframe before the load event) another reference #51003

Testing Instructions

1. Go to the Patterns page in the WordPress admin.
2. Interact with the "Sort by" or "Order by" dropdown to change the sorting order.
3. Verify that no errors appear in the browser console.
4. Confirm that the dropdown functionality works as expected.

Screenshots or screencast

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Co-authored-by: devansh016 <[email protected]>

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