[FIX] stock_picking_batch: do not lock batch model on transient model

closes odoo/odoo#29722

addons/stock_picking_batch/wizard/stock_picking_to_batch.py

@@ -8,7 +8,7 @@ class StockPickingToBatch(models.TransientModel):
     _name = 'stock.picking.to.batch'
     _description = 'Add pickings to a batch picking'
 
-    batch_id = fields.Many2one('stock.picking.batch', string='Batch Picking', required=True, oldname="wave_id")
+    batch_id = fields.Many2one('stock.picking.batch', string='Batch Picking', oldname="wave_id")
 
     @api.multi
     def attach_pickings(self):

addons/stock_picking_batch/wizard/stock_picking_to_batch_views.xml

@@ -8,7 +8,7 @@
         <field name="arch" type="xml">
             <form string="Add pickings to batch">
                 <separator string="Select a batch"/>
-                <field name="batch_id" class="oe_inline" options="{'no_create_edit': True, 'no_open': True}"/>
+                <field name="batch_id" class="oe_inline" required="1" options="{'no_create_edit': True, 'no_open': True}"/>
                 <footer>
                     <button name="attach_pickings" type="object" string="Add to Batch" class="btn-primary"/>
                     <button string="Cancel" class="btn-default" special="cancel"/>