Fatal PHP error

plugin version 2.0.10
WP: 5.8.1
PHP: 8

Going to plugin settings in admin and I get:

Warning: Undefined variable $settings_tabs_field in /home/hatshop/public_html/wp-content/plugins/testimonial/includes/functions-settings-hook.php on line 49

Fatal error: Uncaught Error: Call to a member function generate_field() on null in /home/hatshop/public_html/wp-content/plugins/testimonial/includes/functions-settings-hook.php:49 Stack trace: #0 /home/hatshop/public_html/wp-includes/class-wp-hook.php(303): testimonial_settings_content_general(Array) #1 /home/hatshop/public_html/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array) #2 /home/hatshop/public_html/wp-includes/plugin.php(470): WP_Hook->do_action(Array) #3 /home/hatshop/public_html/wp-content/plugins/testimonial/includes/menu/settings.php(124): do_action('testimonial_set...', Array) #4 /home/hatshop/public_html/wp-content/plugins/testimonial/includes/class-settings.php(22): include('/home/hatshop/p...') #5 /home/hatshop/public_html/wp-includes/class-wp-hook.php(303): class_testimonial_settings->testimonial_settings('') #6 /home/hatshop/public_html/wp-includes/class-wp-hook.php(327): WP_Hook->apply_filters('', Array) #7 /home/hatshop/public_html/wp-includes/plugin.php(470): WP_Hook->do_action(Array) #8 /home/hatshop/public_html/wp-admin/admin.php(259): do_action('testimonial_pag...') #9 /home/hatshop/public_html/wp-admin/edit.php(10): require_once('/home/hatshop/p...') #10 {main} thrown in /home/hatshop/public_html/wp-content/plugins/testimonial/includes/functions-settings-hook.php on line 49

Thanks,
Rudolf