Fatal Error on PHP8?

Hello, turning on PHP 8.0 throws following lines, thank you for checking:
Best wishes!

Current Version is:
- PHP 7.4 -> want to change to 8.0
- WP 6.1.1
- Fortuna 1.91

Notice: Only variables should be passed by reference in /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-content/themes/fortuna/includes/ext/option-tree/ot-loader.php on line 98 Notice: Only variables should be passed by reference in /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-content/themes/fortuna/includes/ext/option-tree/ot-loader.php on line 326 Fatal error: Uncaught ArgumentCountError: Too few arguments to function WP_Widget::__construct(), 0 passed in /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-widget-factory.php on line 62 and at least 2 expected in /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-widget.php:163 Stack trace: #0 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-widget-factory.php(62): WP_Widget->__construct() #1 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/widgets.php(115): WP_Widget_Factory->register() #2 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-content/themes/fortuna/includes/boc_widgets.php(5): register_widget() #3 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-hook.php(308): boc_load_widgets() #4 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters() #5 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/plugin.php(517): WP_Hook->do_action() #6 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/widgets.php(1854): do_action() #7 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-hook.php(308): wp_widgets_init() #8 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters() #9 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/plugin.php(517): WP_Hook->do_action() #10 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-settings.php(617): do_action() #11 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-config.php(91): require_once(‘…’) #12 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-load.php(50): require_once(‘…’) #13 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-admin/admin.php(34): require_once(‘…’) #14 /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-admin/tools.php(40): require_once(‘…’) #15 {main} thrown in /mnt/web111/e2/05/570105/htdocs/WordPress_01/wp-includes/class-wp-widget.php on line 163