Use is_using_gutenberg() wrapper for use_block_editor_for_post().

[peterwilsoncc]

This ensure against fatal errors in versions of WordPress prior to version 6.1.0. The function is only available within wp-admin in these version of WordPress.

Description of the Change

Use is_using_gutenberg() in place of use_block_editor_for_post() to avoid fatal errors in older versions of WordPress.

Closes #1055

How to test the Change

1. Install a version of WordPress between 5.7 and 6.0
2. Activate Distributor
3. Write a post in the block editor
4. Attempt to save the post
5. On this branch the post ought to save, on develop the post will not save.

Changelog Entry

Fixed - Fatal error in WordPress versions 6.0 and earlier.

Credits

Props @peterwilsoncc, @ravinderk

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[ravinderk]

@peterwilsoncc it is working as expected.

I want to suggest two minor changes to syndicatable function. This is not a blocker for this pull request.

• Return bool on line 86
• If we are accessing global $post then define it at the beginning of the function.

[peterwilsoncc]

@ravinderk I've pushed a couple of commits making those changes. This has dismissed your review so are you able to reapprove?