How to Avoid No Thumbnail Issue While Sharing Post on Facebo

function insert_image_src_rel_in_head() {
	global $post;
	if ( !is_singular()) //if it is not a post or a page
		return;
	if(!has_post_thumbnail( $post->ID )) { //the post does not have featured image, use a default image
		$default_image="http://example.com/image.jpg"; //replace this with a default image on your server or an image in your media library
		echo '<meta property="og:image" content="' . $default_image . '"/>';
	}
	else{
		$thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'medium' );
		echo '<meta property="og:image" content="' . esc_attr( $thumbnail_src[0] ) . '"/>';
	}
	echo "
";
}
add_action( 'wp_head', 'insert_image_src_rel_in_head', 5 );