Customizer causing critical error on WP 5.5.1/AST 3.1

Hi,

After updating WP and the AST plugin, the email customizer causes a critical error. Here follows the logs from nginx-php:

2020/09/02 09:58:44 [error] 55846#0: *71164 FastCGI sent in stderr: "PHP message: PHP Warning:  Creating default object from empty value in /var/www/************/wp-content/plugins/woo-advanced-shipment-tracking/includes/customizer/class-wc-email-customizer.php on line 488PHP message: PHP Fatal error:  Uncaught Error: Call to undefined method stdClass::get_content() in /var/www/************/wp-content/plugins/woo-advanced-shipment-tracking/includes/customizer/class-wc-email-customizer.php:499
Stack trace:
#0 /var/www/************/wp-content/plugins/woo-advanced-shipment-tracking/includes/customizer/preview/delivered_preview.php(20): wcast_initialise_customizer_email->preview_delivered_email()
#1 /var/www/************/wp-content/plugins/woo-advanced-shipment-tracking/includes/customizer/class-wc-email-customizer.php(440): include('/var/www/imacch...')
#2 /var/www/************/wp-includes/class-wp-hook.php(287): wcast_initialise_customizer_email->set_up_preview()
#3 /var/www/************/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters()
#4 /var/www/************/wp-includes/plugin.php(544): WP_Hook->do_action()
#5 /var/www/************/wp-includes/class-wp.php(388): do_action_ref_array()
#6 /var/www/************/wp-in" while reading response header from upstream, client: **********, server: ************, request: "GET /?wcast-email-customizer-preview=1&customize_changeset_uuid=0f23111c-a82a-4ca9-92f7-ea7164fd000e&customize_theme=storefront&customize_messenger_channel=preview-4&customize_autosaved=on HTTP/2.0", upstream: "fastcgi://unix:/var/run/php-fpm/php-fpm.sock:", host: "************", referrer: "https://************/wp-admin/customize.php?wcast-customizer=1&email=ast_tracking_display_panel&url=https%3A%2F%2F************%2F%3Fwcast-tracking-preview%3D1&return=https%3A%2F%2F************%2Fwp-admin%2Fadmin.php%3Fpage%3Dwoocommerce-advanced-shipment-tracking%26tab%3Dsettings&autofocus[panel]=ast_tracking_display_panel"
2020/09/02 10:01:34 [error] 55846#0: *71571 access forbidden by rule, client:*******, server: *********, request: "GET / HTTP/1.1", host: **********