gistfile1.txt

//////////////////////

// Find this:

class Charitable_Ambassadors_Email_New_Campaign extends Charitable_Email {

    /**
     * @var     string
     */
    CONST ID = 'new_campaign';
    
    ...
}

//////////////////////

// Replace with this: 

class Charitable_Ambassadors_Email_New_Campaign extends Charitable_Email {

    /**
     * @var     string
     */
    CONST ID = 'new_campaign';
    
    /**
     * Sets whether the email allows you to define the email recipients.
     * 
     * @var     boolean 
     * @access  protected
     * @since   1.1.0
     */
    protected $has_recipient_field = true;
    ...
}