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.

    Luís

    Hi Granite ,

    Hope you're doing well today!

    From what I searched in the support forum, this issue may be related to the outdated version of WordPress, because it was always solved after upgrade WordPress (at least to the 4.7 version).

    I will strongly advise to update WordPress, at least, because it's a security risk.

    However, I am not sure if we can overcome this issue on your WordPress version, so, I would like to flag the SLS team (our code experts ).

    To do this, can you please send us the FTP details of your website, so that the SLS can investigate further? Please send it privately through our contact form:

    https://premium.wpmudev.org/contact/#i-have-a-different-question

    Send in:
    Subject: "Attn: Luis Lopes"
    - WordPress admin username
    - WordPress admin password
    - Login url
    - FTP credentials (host/username/password)
    - Link back to this thread for reference
    - Any other relevant urls/information

    Cheers, Luís