We want to have certain columns in a tree view sorted in an ascending way. In order to achieve it I tried to edit the tree view under Manufacturing Orders (Edit Tree View (Developers Mode)) and I added the following line

<tree default_order='x_Delivery_Date_To_FSLSales'>

It now reads:

<?xml version="1.0"?><tree fonts="bold:message_unread==True" colors="blue:state in ('draft','confirmed');red:date_planned&lt;current_date and state not in ('done','cancel');black:date_planned&gt;=current_date;gray:state in ('done','cancel') " string="Manufacturing Orders">

<tree default_order='x_Delivery_Date_To_FSLSales'>
</tree>

<field name="x_Delivery_Date_To_FSLSales"/>

<field name="sale_ref"> </field>

</tree>

It comes up with a 

ValidateError

Field(s) `arch` failed against a constraint: Invalid view definition

Can someone please help out on how to have a tree view sorted according to a specific column. I have seen various answers but could not figure out how to edit the view definition in order to achieve this.