I am trying to change the events registration page Quantity field drop down box color. I have the file in here

  /odoo/addons/website_event/views/website_event.xml

I get the following error while upgrading the website_event from odoo admin

[view_id: 1141, xml_id: website_event.view_event_sale_form, model: event.event, parent_id: 590]

None" while parsing /home/nirmalasudhir/projects/afbsdemo/afbsdemo/parts/odoo/addons/website_event/views/website_event_backend.xml:5, near<record model="ir.ui.view" id="view_event_sale_form">

I haven't changed anything in the odoo base module (website_event). I am struck with this error.

I am using odoo 9.