Hi,

The best solution is to attribute an xml_id to your picking type in order to call it with reference: self.env.ref(..)

To do that you have to create a ir_model_data record related to the picking type ( it's better and advanced as you need to add it in migrations folder.. change version in manifest to trigger it ...)

you can also try to export the list of picking types and odoo will automatically attribute an xml_id like '__export__.some_text_here' 
then use it in your reference and see if it's going to work.

Hope this helps.