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Props joemcgill, peterwilsoncc, afragen.

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Doesn’t is_block_theme() return false if directories aren’t set up? If so, is there a need to exit early in the warning?

Good question, @afragen! Returning early ensures that wp_get_theme() is avoided to avoid the bug from Trac-63062.

(Edit: also, FWIW, I checked the Core Rollback plugin, and that plugin is calling wp_is_block_theme() as part of the settings that are triggered on admin_init, so no notice should be thrown.)

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@joemcgill I didn't think there was an issue in Core Rollback, but it is an example of use in a plugin.

I've added your changes in 2 of my test sites running 6.8-beta2. I think there might still be an issue somewhere in The Events Calendar plugin.

I've added your changes in 2 of my test sites running 6.8-beta2. I think there might still be an issue somewhere in The Events Calendar plugin.

Thanks for testing, @afragen! You're right, this will get triggered on TEC, because they are calling wp_is_block_theme() on the plugins_loaded hook in order to determine whether the current theme supports full site editing. While calling wp_is_block_theme() this early won't cause the bug from https://core.trac.wordpress.org/ticket/63062 to occur, it will result in incorrect behavior, because it will return false even if the active theme is a block theme, since the theme has not yet been set up.

The changes have been running on 2 of my test servers and only reported errors are from The Events Calendar and Events Calendar Pro.

It’s looking much better.

I've merged in Joe's suggestion of switching to test for the definition of theme directories. No error is thrown in the customizer during my testing.

Using a modified version of @SirLouen's testing plugin while running @fabiankaegy's MVP block child theme attached to Core-63062, I've been able to confirm child theme are working as expected. Without the early return prior to the the directories being set up, the child theme continues to result in a fatal error when the function is called on muplugins_loaded.

Coincidently, I've got some client work to do this-afternoon on a site running WooCommerce, so I will check for notices in that once I have switched tasks.

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 60037
GitHub commit: 79fd25f

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.

Replying to @peterwilsoncc

No error is thrown in the customizer during my testing.

I got this error yesterday in the morning and this is why I started researching further in the idea proposed by dinhtungdu.
Then I built the plugin in the evening, tested everything according to his suggested ideas, and thereafter, tested again this patch, and got the error once more.

But I was testing against the if ( ! did_action( 'setup_theme' ) ) { version

Not the current if ( empty( $GLOBALS['wp_theme_directories'] ) ) { version, which is entirely different. But my plugin cannot catch this.

PS: I've noted that the convention to check for the global $wp_theme_directories is using this format:

	global $wp_theme_directories;
	if ( empty( $wp_theme_directories ) ) {
		_doing_it_wrong( __FUNCTION__, __( 'This function should not be called before themes are set up.' ), '6.8.0' );
		return false;
	}

Instead of $GLOBALS['wp_theme_directories'] that I observe it's only used in the testing files, despite both forms are right