Deprecated code and not compatible with WP 5

Unfortunately this became unstable after i upgraded to WP 5, rolling back to 4.9.13

Your plugin has become unstable and is throwing cricital errors during different actions. I sent this to your support team aswell. Users have been complaining for a while. Will edit review after a stable version has been released.

WordPress versie 5.5.1
Huidige plugin: Ninja Forms (versie 3.4.30)
PHP versie 7.4.10

Foutdetails
===========
Een fout van het type E_ERROR werd veroorzaakt op regelnummer 324 van het bestand /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-content/plugins/ninja-forms/deprecated/classes/notifications.php. Foutmelding: Uncaught Error: Call to a member function save_admin() on null in /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-content/plugins/ninja-forms/deprecated/classes/notifications.php:324

Stack trace:
#0 /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-content/plugins/ninja-forms/deprecated/includes/admin/save.php(109): NF_Notifications->save_admin(354, Array)
#1 /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-includes/class-wp-hook.php(287): ninja_forms_admin_save('')
#2 /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(false, Array)
#3 /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#4 /home/DOMAIN/domains/DOMAIN.nl/public_html/wp-admin/admin.php(175): do_action('admin_init')
#5 {main}
thrown

Looking forward using this again without trouble because i realy liked it.