There are something wrong with test data. Please try to create PO with Vender ZiddlemanInc: Ziddleman & Sons Suppliers Inc and Customer ZIRET: Ziziwork Retail & Wholesale.
If I press “Create Purchase Order” and then “Create” on the default customer (55900: Test Vendor) it takes me to a new order screen with the Order Detail tab showing:
Cannot get property ‘vendorPartyId’ on null object