Nobody can answer this:

Why stock_picking_id field is not populated with the calling ID? (It is being called from a button in stock_picking form which launches the popup tree view):

# -*- coding: utf-8 -*-

from openerp import models, fields, api

class LabelPicking(models.TransientModel):
        _name = "label.picking"
        stock_picking_id = fields.Many2one(
                'stock.picking',
                required=True,
                default=lambda self: self.env.context['active_id'],
        )

XML:

<?xml version="1.0" encoding="utf-8" ?>

<openerp>

        <data>

                <record id="view_tree_label_picking" model="ir.ui.view">

                        <field name="name">Labels Dispatch Items Tree</field>

                        <field name="model">label.picking</field>

                        <field name="arch" type="xml">

                                <tree>

                                        <field name="stock_picking_id"/>

                                </tree>

                        </field>

                </record>

        </data>

</openerp>