protected $params;
/**
- * @param callable $callback
+ * @param callable|null $callback
* @param array $params Callback arguments (since 1.25)
* @throws Exception
*/
public function __construct( $callback, array $params = array() ) {
- if ( !is_callable( $callback ) ) {
+ if ( $callback !== null && !is_callable( $callback ) ) {
throw new InvalidArgumentException( "Provided callback is not valid." );
}
$this->callback = $callback;