We're running big tree 4.5.7 on a couple sites. Client reported the Active 404s returned empty, possibly after th most recent update, I haven't been keeping an eye on this specifically.

Seems that there was a DB update at some point that failed to run, as our bigtree_404s table retained the "NULL" for redirect_url column.
DB structure:
CREATE TABLE `bigtree_404s` (
  `id` int UNSIGNED NOT NULL,
  `broken_url` varchar(1024) DEFAULT NULL,
  `get_vars` varchar(1024) DEFAULT NULL,
  `redirect_url` varchar(1024) DEFAULT NULL,
  `requests` int UNSIGNED NOT NULL DEFAULT '0',
  `ignored` char(2) NOT NULL DEFAULT '',
  `site_key` varchar(255) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb3;

Ran the following queries not he DB table to correct:
// update existing records to avoid null values
UPDATE `bigtree_404s` SET `redirect_url` = '' WHERE `redirect_url` IS NULL;

// update structure to not have a null on that column
ALTER TABLE `bigtree_404s` CHANGE `redirect_url` `redirect_url` VARCHAR(1024) CHARACTER SET utf8mb3 COLLATE utf8mb3_general_ci NOT NULL;

Just FYI in case someone else runs into this. Not sure why our production server would fail on this update.