added some error checking if class/field descriptors not found in model
added some error checking if class/field descriptors not found in model



Former-commit-id: c79334c83711692ce46ed32ad952965520cacedc
julie-sullivan/phytomine,julie-sullivan/phytomine,julie-sullivan/phytomine,julie-sullivan/phytomine,julie-sullivan/phytomine,julie-sullivan/phytomine,julie-sullivan/phytomine