Undefined index and variables with WordPress 3.8.1

Upon installation of the plugin I receive this annoying output:

Notice: Undefined index: force_upgrade in /var/www/vhosts/mywebsite.org/httpdocs/wp-content/plugins/sp-client-document-manager/classes/install.php on line 7 Notice: Undefined variable: alters in /var/www/vhosts/mywebsite.org/httpdocs/wp-content/plugins/sp-client-document-manager/classes/install.php on line 113

WordPress suggests to disable the plugin with the common:

The plugin generated 154 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

That output string appears all over the place on my pages.
How do I fix this?

Regards.

https://wordpress.org/plugins/sp-client-document-manager/