Error when creating Add Access Authority

[spark-green]

I am trying to use this module (master branch) with silverstripe version 3.2. I am getting bellow error.

[Recoverable Error] Method MultiValueField::__toString() must return a string value
GET /admin/settings/EditForm/field/AccessAuthority/item/new/

Line 173 in /framework/forms/DropdownField.php

Also I am struggling how to publish the page? Their is no publish button visible to the authorized user.