Error after publishing post.

Fatal error: Uncaught Error: Non-static method Gravitecnet_Admin::set_gravitec_error_transient() cannot be called statically in /home/u877128135/domains/thefactnews.in/public_html/wp-content/plugins/gravitec-net-web-push-notifications/admin/class-gravitecnet-admin.php:831 Stack trace: #0 /home/u877128135/domains/thefactnews.in/public_html/wp-content/plugins/gravitec-net-web-push-notifications/admin/class-gravitecnet-admin.php(682): Gravitecnet_Admin::send_gravitec_notification_on_post() #1 /home/u877128135/domains/thefactnews.in/public_html/wp-includes/class-wp-hook.php(324): Gravitecnet_Admin::on_publish_post() #2 /home/u877128135/domains/thefactnews.in/public_html/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters() #3 /home/u877128135/domains/thefactnews.in/public_html/wp-includes/plugin.php(517): WP_Hook->do_action() #4 /home/u877128135/domains/thefactnews.in/public_html/wp-includes/post.php(5740): do_action() #5 /home/u877128135/domains/thefactnews.in/public_html/wp-includes/post.php(5000): wp_transition_post_status() #6 /home/u877128135/domains/thefactnews.in/public_html/wp-includes/post.php(5212): wp_insert_post() #7 /home/u877128135/domains/thefactnews.in/public_html/wp-admin/includes/post.php(453): wp_update_post() #8 /home/u877128135/domains/thefactnews.in/public_html/wp-admin/post.php(227): edit_post() #9 {main} thrown in /home/u877128135/domains/thefactnews.in/public_html/wp-content/plugins/gravitec-net-web-push-notifications/admin/class-gravitecnet-admin.php on line 831