[FIX] stock: scheduled date read-only on picking

- Create a picking
- At creation, add a product and change the scheduled date

The scheduled date is reset to its original value when saving.

This is a standard behavior of the ORM for computed fields. Therefore we
simply forbid the modification of the scheduled date at creation.

opw-807572

addons/stock/views/stock_picking_views.xml

@@ -200,6 +200,7 @@
             <field name="arch" type="xml">
                 <form string="Transfer">
                 <header>
+                    <field name="id" invisible="1"/>
                     <field name="is_locked" invisible="1"/>
                     <field name="show_mark_as_todo" invisible="1"/>
                     <field name="show_check_availability" invisible="1"/>
@@ -245,7 +246,7 @@
                             <field name="backorder_id" readonly="1" attrs="{'invisible': [('backorder_id','=',False)]}"/>
                         </group>
                         <group>
-                            <field name="scheduled_date"/>
+                            <field name="scheduled_date" attrs="{'readonly': [('id', '=', False)]}"/>
                             <field name="origin" placeholder="e.g. PO0032"/>
                             <field name="owner_id" groups="stock.group_tracking_owner"/>
                             <div groups="stock.group_tracking_owner" colspan="2" col="2">