problem with php 8

hi, i’m working to migrate to php 8 but your plugin sends me

[16-Jan-2021 11:25:21 UTC] PHP Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($function) must be a valid callback, non-static method UserOnline_WpStats::page_general_stats() cannot be called statically in /home/xxx/public_html/wp-includes/class-wp-hook.php:287
Stack trace:
#0 /home/xxx/public_html/wp-includes/plugin.php(212): WP_Hook->apply_filters(‘<h2 id=”General…’, Array)
#1 /home/xxx/public_html/wp-content/plugins/wp-stats/wp-stats.php(417): apply_filters(‘wp_stats_page_p…’, ‘<h2 id=”General…’)
#2 /home/xxx/public_html/wp-content/plugins/wp-stats/wp-stats.php(62): stats_page()
#3 /home/xxx/public_html/wp-includes/class-wp-hook.php(287): display_stats(”)
#4 /home/xxx/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters(”, Array)
#5 /home/xxx/public_html/wp-includes/plugin.php(484): WP_Hook->do_action(Array)
#6 /home/xxx/public_html/wp-admin/admin.php(259): do_action(‘dashboard_page_…’)
#7 /home/xxx/public_html/wp-admin/index.php(10): require_once(‘/home/xxx/…’)
#8 {main}
thrown in /home/xxx/public_html/wp-includes/class-wp-hook.php on line 287

in plugin window

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