Portlet importexport throws Exception when a Choice field is empty

[nngu6036]

Steps to reproduce:

• Create a new Plone Site
• Go to ZMI -> portal_setup
• Export the potlet to a tar ball
• Import the same tar ball

The system throws exception,

ConstraintNotSatisfied: (u'', 'root_uid')

I put the debug and found that it breaks to the assignment for navigation portlet. Somehow the Choice field is empty and is exported as an XML element with no value, but the import process does not accept it.

I have a work around by removing the XML element manually from portlets.xml.