Presence of 'is_child' causes warnings on non-post pages and new, unsaved posts

Hello,

First of all, thank you for the excellent plugin. It's extremely useful.

While attempting to exclude a MetaBox if a page is the child of another page I encountered the following warning:

Notice: Trying to get property of non-object in /home/ubuntu/app/wp/wp-content/plugins/meta-box-include-exclude/class-mb-include-exclude.php on line 301

This seems to be due to the fact that the $post object assumes that there will always be a post_id present. Unfortunately, (as far as I understand) new posts do not have their post_id set until saving and other areas of the Dashboard do not have post_id. This results in the notice being displayed.

The following is one possible way to get around this issue, though there are likely other ways to approach a solution:

	public static function check_is_child( $value )
	{
		$post     = get_post( self::$post_id );
		if ( is_object($post) ){
			$is_child = $post->post_parent ? true : false;
			return $is_child === $value;
		} else {
			return false;
		}
	}