fatal error after the update

Hi
Just to notify you that your recent update of the plugin has caused a fatal error on my website which is using the plugin for months now.
I think there is some bug you have not spotted.
I had to uninstall the plugin to get the site back to normal.
The following error information is for your reference.

WordPress version 5.7.2
Current theme: Corpobrand (version 1.0.5)
Current plugin: Modern Events Calendar Lite (version 5.21.0)
PHP version 7.0.33-0+deb9u10

Error Details
=============
An error of type E_ERROR was caused in line 64 of the file /var/www/ssldocs/wp-content/plugins/modern-events-calendar-lite/app/core/src/Base.php. Error message: Uncaught TypeError: Return value of MEC\Base::define() must be an instance of MEC\void, none returned in /var/www/ssldocs/wp-content/plugins/modern-events-calendar-lite/app/core/src/Base.php:64
Stack trace:
#0 /var/www/ssldocs/wp-content/plugins/modern-events-calendar-lite/app/core/src/Base.php(29): MEC\Base->define()
#1 /var/www/ssldocs/wp-content/plugins/modern-events-calendar-lite/app/core/src/Base.php(44): MEC\Base->__construct()
#2 /var/www/ssldocs/wp-content/plugins/modern-events-calendar-lite/app/core/mec.php(19): MEC\Base::instance()
#3 /var/www/ssldocs/wp-content/plugins/modern-events-calendar-lite/modern-events-calendar-lite.php(43): require_once(‘/var/www/ssldoc…’)
#4 /var/www/ssldocs/wp-settings.php(391): include_once(‘/var/www/ssldoc…’)
#5 /var/www/ssldocs/wp-config.php(77): require_once(‘/var/www/ssldoc…’)
#6 /var/www/ssldocs/wp-load.php(37): require_once(‘/var/www/ssldoc…’)
#7 /var/www/ssldocs/wp-admin/admin.php(34): require_once(‘/var/www/ssldoc…’)
#8 /var/www/ssldocs/wp-admin/index.`