Handling shipment exception on Purchase Order results in KeyError
When trying to use "HandleShipmentException" wizard on Purchase, the "transition_handle" method results in KeyError.
How to reproduce:
1. Create a Purchase Order (with one purchase line to follow)
2. Process the purchase order
3. Create a ShipmentIn with moves created by purchase order
4. Receive the shipment partially (Important: Do not receive the entire quantity of purchase order line)
5. Mark the shipment as "done"
6. Now, the "shipment_state" on Purchase Order will be "exception".
7. Goto HandleShipmentException Wizard and do not select any moves to recreate (this step is important).
8. Clicking "Ok" on the wizard will result in KeyError
Crashes at: https://github.com/tryton/sale_supply_drop_shipment/blob/develop/purchase.py#L210