This error can occur when you are importing requirements into Spira from HP ALM and you have custom fields defined in ALM on the requirements.
This is actually a known bug in ALM that causes the HP ALM API to not allow you to access the custom properties.
According to Micro Focus users:
One thing to check - make sure the new field is also added to the Undefined requirement type. I know not having a requirement field visible for that type could cause some strange behavior in previous versions.
This basically means that you need to assign the requirement custom fields to the 'Undefined' type as well as the various named types inside the ALM Site Admin.