Segmentation fault through plugin client

A response of grpc calls within methods of DeviceModelClient is nil as soon as an error occured within the call.

Description

In some methods values of the response were directly accessed and returned. This leads to a segmentation fault if the response is nil.

Expected Behavior

DeviceModelCLient method calls should not crash the controller

Actual Behavior

Some method calls lead to a segmentation fault and crashing the controller

Possible Fix

Correct error handling