Forbid setting array items to NULL
Forbid setting array items to NULL

At the moment, doing this will crash PyMI with an access
violation error when the according instance object is
serialized.

We'll simply throw an exception if this is requested.

cloudbase/PyMI,cloudbase/PyMI,alinbalutoiu/PyMI,alinbalutoiu/PyMI,alinbalutoiu/PyMI,cloudbase/PyMI