fixes improper exception handling for NULL parent.  I thought root model nodes had themselves as their parent - apparently, they now have been changed to hold a NULL parent.
fixes improper exception handling for NULL parent.  I thought root model nodes had themselves as their parent - apparently, they now have been changed to hold a NULL parent.

gidden/cyclus,mbmcgarry/cyclus,gidden/cyclus,mbmcgarry/cyclus,gidden/cyclus,hodger/cyclus,hodger/cyclus,hodger/cyclus,rwcarlsen/cyclus,rwcarlsen/cyclus,Baaaaam/cyclus,mbmcgarry/cyclus,rwcarlsen/cyclus,rwcarlsen/cyclus,Baaaaam/cyclus,gidden/cyclus,mbmcgarry/cyclus,Baaaaam/cyclus,hodger/cyclus,hodger/cyclus