The module is throwing a ValueError: <type 'exceptions.AttributeError'>: "'NoneType' object has no attribute 'x_pt_tip_rate'" when confirming the purchase order.  When the scope of the tax is set to "sales" then the module works fine.  

The attribute is associated to the product module which can be retrieved when creating the sales order and request for quotation.  It's not until I confirm the purchase order does the error occur.  

I'm using odoo 10, online version.  The latest module version: 10.saas~15.1.0