WARNING: CALL USER FUNC ARRAY !!

Hello, thanks for this great plugin!
I’m having the following error after updating your plugin!

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘wpmem_admin_options’ not found or invalid function name in /home/customer/www/MYSITE.com/public_html/wp-includes/class-wp-hook.php on line 307

What could be causing this error? and How can I solve this?

Thanks for your time,
Regards,