Please try unloading StructureUpgrade, deleting the configuration and data file (if there is one) then loading StructureUpgrade.
o.unload StructureUpgrade
o.load StructureUpgrade
If this doesn't resolve the issue, can you confirm if plugin functionality works correctly? I believe this is a harmless error that won't interrupt plugin functionality. If it is obstructing plugin functionality I'll work on a fix immediately, otherwise I'll investigate and release a patch later this week.