We have tested this issue with the latest version of the API and following sample code at our end, but couldn’t notice any Null Reference Exception as you have mentioned. Please try it with the latest version of the API and let us know if you face any issue.
Sample Code:
Notebook notebook = new Notebook(“780448\Open Notebook.OneToc2”);
The request for loading notebook from stream has been logged as NOTENET-2151 in our issue tracking system. Our Product team will consider it for implementation and we’ll update you here once there is some information available in this regard.