Bug in metabox.php on line 52

Line 52 should be:

if( isset($_POST['pw_spe_expiration_nonce']) && ! wp_verify_nonce( $_POST['pw_spe_expiration_nonce'], 'pw_spe_edit_expiration' ) ) {
		return;
	}

isset($_POST['pw_spe_expiration_nonce']) needs to be added.

https://www.ads-software.com/plugins/simple-post-expiration/