Call to a member function add_cap() on null

My site upgraded to version 3.0.2 overnight, and trying to load the site this morning gave the error:

[09-Jan-2021 09:15:03 UTC] PHP Fatal error: Uncaught Error: Call to a member function add_cap() on null in /var/www/* my website */wordpress/wp-content/plugins/publishpress/modules/editorial-metadata/editorial-metadata.php:415
Stack trace:
#0 /var/www/* my website */wordpress/wp-content/plugins/publishpress/modules/editorial-metadata/editorial-metadata.php(227): PP_Editorial_Metadata->setDefaultCapabilities()
#1 /var/www/* my website */wordpress/wp-content/plugins/publishpress/publishpress.php(463): PP_Editorial_Metadata->upgrade(‘3.0.1’)
#2 /var/www/* my website */wordpress/wp-includes/class-wp-hook.php(287): publishpress->action_ini_for_admin(”)
#3 /var/www/* my website */wordpress/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(NULL, Array)
#4 /var/www/* my website */wordpress/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#5 /var/www/* my website */wordpress/wp-settings.php(557): do_action(‘init’)
#6 /var/www/* my website */wordpress/wp-config.php(149): require_once(‘/var/www/* my website */word…’)
#7 /var/www/* my website */wordpress/wp-load.php(37): require_once(‘/var/www/* my website */word…’)
#8 /var/www/* my website */wordpress/wp-cron.php(44): require_once(‘/var/www/* my website */wor in /var/www/* my website */wordpress/wp-content/plugins/publishpress/modules/editorial-metadata/editorial-metadata.php on line 415

I’ve downgraded to 3.0.1. The error also broke wp-cli so I had to rename the directory, and then reinstall 3.0.1