PHP Fatal error: Class 'WP_Ajax_Upgrader_Skin'

Hello,

I try to install WP Smush Pro plugin and I got this error:
PHP Fatal error: Class 'WP_Ajax_Upgrader_Skin' not found in /data/sites/lscconnect.com/web/wp-content/plugins/wpmudev-updates/includes/class-wpmudev-dashboard-upgrader.php on line 634, referer: http://lscconnect.com/wp-admin/admin.php?page=wpmudev-plugins

I'm using WordPress version 4.5. Unfortunately I can't update the core because of one the plugins I'm using.

Can you please help me?

Thanks,
Andrei.