Undefined Index error

[peter-mumford]

I've installed this plugin on my network, but I get errors.
For every option I don't check, I get an error like this:
Notice: Undefined index: allow_email_addresses in /Users/peter/network/wp-content/plugins/wp-plugin-username-restriction-override/network-username-restrictions-override.php on line 230

I'm using WP 4.9.8, and PHP 7.1.19