Global Search undefined global post id fix

<?php
add_filter( 'body_class', 'bosschild_events_page_class', 9 );
function bosschild_events_page_class( $classes ){
    if( !function_exists( 'boss_events_page_class' ) )
        return $classes;

    remove_filter( 'body_class', 'boss_events_page_class' );

    global $post;
    $events_page_id = get_option ( 'dbem_events_page' );

    if( isset( $post->ID ) && $post->ID == $events_page_id ) {
        $classes[] = 'events-page';
    }

    if( isset( $post->ID ) && $post->ID == $events_page_id && get_option('dbem_display_calendar_in_events_page')) {
        $classes[] = 'fullcalendar-page';
    }
    return array_unique( $classes );
}