PartShop.submit fails when document.displayId has no value

[tramyn]

I am getting the following error when submitting contents to SynBioHub when calling PartShop.submit(). Below is a snippet of this error:

if len(doc.displayId) > 0:
TypeError: object of type 'NoneType' has no len()

Parameters pass to PartShop.submit() looks something like this:

PartShop.submit(document, collection, flags)

document represents an SBOL document object
collection is set to uri from SynBioHub
flags is set to 2