Received this from WordPress after the error, and thought it might be helpful.
WordPress version 6.1.1
Active theme: Astra (version 4.0.2)
Current plugin: JoomSport Achievements (version 1.5.1) PHP version 8.1.13
Error Details
=============
An error of type E_ERROR was caused in line 395 of the file **********/wp-content/plugins/joomsport-achievements/includes/meta-boxes/joomsport-achievments-meta-stage.php. Error message: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in **********/wp-content/plugins/joomsport-achievements/includes/meta-boxes/joomsport-achievments-meta-stage.php:395
Stack trace:
#0 **********/wp-content/plugins/joomsport-achievements/includes/meta-boxes/joomsport-achievments-meta-stage.php(330): JoomSportAchievmentsMetaStage::saveMetaResults()
#1 **********/wp-includes/class-wp-hook.php(310): JoomSportAchievmentsMetaStage::joomsport_stage_save_metabox()
#2 **********/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()
#3 **********/wp-includes/plugin.php(517): WP_Hook->do_action()
#4 **********/wp-includes/post.php(4717): do_action()
#5 **********/wp-includes/post.php(4819): wp_insert_post()
#6 **********/wp-admin/includes/post.php(426): wp_update_post()
#7 **********/wp-admin/post.php(227): edit_post()
#8 {main}
? thrown