Missing argument in add_theme_support( 'editor-font-sizes' ) displays PHP warning in WP 5.9

[chthonic-ds]

Description

Updating to WP 5.9 on a site using a classic theme without a theme.json file, I see the following PHP warning:

Warning: Invalid argument supplied for foreach() in \wp-includes\class-wp-theme-json.php on line 1949

Font sizes for this theme are managed in CSS, so the following theme supports have been set to prevent font size controls being exposed in the editor:

add_theme_support( 'editor-font-sizes' ); // Prevents default font sizes being used.
add_theme_support( 'disable-custom-font-sizes' );

The warning appears because an array is expected as an argument of the editor-font-sizes support:

add_theme_suport( 'editor-font-sizes', array() );

Testing WP prior to 5.9, passing an argument was not previously required to prevent a warning. A google search returns a couple of tutorial-style sites not setting an empty array, suggesting there might be more of this configuration out there.

In this case it's not a problem applying a fix at the theme level, but to help prevent other themes being affected how about performing a check in class-wp-theme-json.php?

Checking an array exists prevents the notice:

if ( isset( $settings['fontSizes'] ) && is_array( $settings['fontSizes'] ) ) { // Line 1946 of class-wp-theme-json.php

Step-by-step reproduction instructions

1. Add the following to the functions.php of a theme running on WP 5.9:

function yvxasrfazh_disable_editor_font_sizes() {
	add_theme_support( 'editor-font-sizes' );
	add_theme_support( 'disable-custom-font-sizes' );
}

add_action( 'after_setup_theme', 'yvxasrfazh_disable_editor_font_sizes' );

2. Load the site, observe a PHP warning.
3. Add an empty array as an argument to the editor-font-sizes support.
4. Load the site, see the PHP warning has disappeared

Screenshots, screen recording, code snippet

No response

Environment info

• WordPress 5.9, any theme

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[kathrynwp]

Thanks for the report and the PR!

Noting that I can still replicate this issue with:

WordPress 6.0.1
No plugins active
Theme: Twenty Twenty-One
PHP version 7.3.8

[t-hamano]

Update: I have confirmed that this problem still exists in WordPress 6.1.1, whether or not the latest Gutenberg plugin is enabled.