nil pointer deference when k8s prematurely shuts down core

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xc71219]goroutine 1 [running]:
github.com/opencord/voltha-go/common/grpc.(*GrpcServer).Stop(...)
    /go/src/github.com/opencord/voltha-go/common/grpc/server.go:121
github.com/opencord/voltha-go/rw_core/core.(*Core).Stop(0xc00024a6e0, 0x114c500, 0xc000206900)
    /go/src/github.com/opencord/voltha-go/rw_core/core/core.go:141 +0x89
main.(*rwCore).stop(0xc000117680, 0x114c500, 0xc000206900)
    /go/src/github.com/opencord/voltha-go/rw_core/main.go:170 +0x92
main.main()
    /go/src/github.com/opencord/voltha-go/rw_core/main.go:291 +0x750