Getting user roles in plugin files

Figured my comment would actually make a better answer, so here we go.

You can hook other methods within the plugin class onto admin_init (for the admin side) within the constructor, like so:

class Plugin_Class {

    public function __construct() {
        add_action( 'admin_init', array( $this, 'some_other_method' ) );
    }

    public function some_other_method() {
        // do something fancy with get_editable_roles()
    }
}

This is generally better practice, along with hooking onto init for front end stuff, rather than directly calling things from the constructor as it allows everything to load up properly first. Also, other developers can remove the hook if they are using your plugin and want to extend/modify it, without having to directly hack the file. Essentially, your constructor is just a big set of init hooks, or a single hook to a separate initialization method.

Also, keep in mind get_editable_roles() is only loaded in admin areas, where is_admin() would evaluate to true. You might think about putting a check for that somewhere in the plugin too before you attempt to use the editable roles function.