PHP Warning when $this->push_syndicate_settings is still set to null

[terriann]

Description:

There is an assumption that the init hook will run before the cron_schedules hook, but if another plugin, the theme or an mu-plugin calls the function wp_get_schedules() before the the init hook fires (typically by calling it outside of the context of any hooks), the following warning gets logged:

PHP message: Warning: Trying to access array offset on value of type null in /var/www/wp-content/plugins/push-syndication/includes/class-wp-push-syndication-server.php on line 1227

The cause is $this->push_syndicate_settings being null when WP_Push_Syndication_Server::cron_add_pull_time_interval() is called

syndication/includes/class-wp-push-syndication-server.php

Lines 1224 to 1230 in b47a461

	public function cron_add_pull_time_interval( $schedules ) {
	// Adds the custom time interval to the existing schedules.
	$schedules['syn_pull_time_interval'] = array(
	'interval' => intval( $this->push_syndicate_settings['pull_time_interval'] ),
	'display' => __( 'Pull Time Interval', 'push-syndication' )
	);

As far as I can tell this does not create a functional problem with the Syndication plugin, but the warning can cause noise in logs which can make it more difficult to find important log messages.

Workaround:

As a workaround, a defensive check can be added to the plugin:

	public function cron_add_pull_time_interval( $schedules ) {

+		// Only add custom interval if syndication settings are defined.
+		if (
+  		  empty( $this->push_syndicate_settings )
+		  || ! array_key_exists( 'pull_time_interval', $this->push_syndicate_settings )
+		) {
+			return $schedules;
+		}
+
		// Adds the custom time interval to the existing schedules.
		$schedules['syn_pull_time_interval'] = array(
			'interval' => intval( $this->push_syndicate_settings['pull_time_interval'] ),
			'display' => __( 'Pull Time Interval', 'push-syndication' )
		);

		return $schedules;

	}

Steps to Reproduce:

This can be reproduced when installing the Syndication plugin on a plain WordPress installation and calling the wp_get_schedules(); outside of any hooks in the default theme's functions.php. Then load any front or backend page and the error will be logged.