Fix default get of wizard 'Create Agreement Purchase Order'

Wizard failed sometimes on startup when called from Logistics Requisition Line
This was due to a browse of LR Source using ids of LR Line from
active_ids
[REF] Add a check if active_model is not valid for Wizard of PO creation from LRS
Fix tests by adding active_model for wizard
Merge pull request #76 from yvaucher/8.0-fix-fa-create-po-default_get

Fix default get of wizard 'Create Agreement Purchase Order'