WP in Mono-site, Undefined array key "id_column_exists" warning log

[f-leclere]

Hi,
id_column_exists variable is not defined when wordpress is in mono-site mode.

Warning: Undefined array key "id_column_exists" in //wp-native-php-sessions/pantheon-sessions.php on line 562

My proposal is to modify pantheon-sessions.php like following

	public function add_index( $start_position ) {
		global $wpdb;

		$output = [
			'no_session_table' => 0,
			'id_column_exists' => 0,
		];
		if ( ! is_multisite() ) {
			$this->safe_output( __( 'Single site detected. Beginning processing... \n', 'wp-native-php-sessions' ), 'log' );

			$this->add_single_index( $wpdb->prefix, $output );

			$this->safe_output( __( 'Operation complete, please verify that your site is working as expected. When ready, run wp pantheon session primary-key-finalize to clean up old data, or run wp pantheon session primary-key-revert if there were issues.', 'wp-native-php-sessions' ), 'log' );

			return;
		}

		$this->safe_output( __( 'Multisite installation detected. Processing Sites individually...', 'wp-native-php-sessions' ), 'log' );

		$site_list = $this->get_all_sites( $start_position );
		$site_count = count( $site_list );

[jazzsequence]

Thanks for reporting this. Tracking internally as BUGS-7105.