Go panic from kn service delete command

[nak3]

Version: 9c21b8d

Steps to produce:

1. Create knative service

(e.g)

$ ./kn service create hello-example --image=gcr.io/knative-samples/helloworld-go 

2. Delete the service by kn service delete command

$ ./kn service delete hello-example 
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x10b00f4]

goroutine 1 [running]:
github.com/knative/client/pkg/kn/commands.NewServiceDeleteCommand.func1(0xc00011fb80, 0xc0003f9800, 0x1, 0x1, 0x0, 0x0)
	/home/knakayam/.go/src/github.com/knative/client/pkg/kn/commands/service_delete.go:44 +0x104
github.com/spf13/cobra.(*Command).execute(0xc00011fb80, 0xc0003f97a0, 0x1, 0x1, 0xc00011fb80, 0xc0003f97a0)
	/home/knakayam/.go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:762 +0x465
github.com/spf13/cobra.(*Command).ExecuteC(0xc000155680, 0x0, 0x0, 0xc000155680)
	/home/knakayam/.go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:852 +0x2ec
github.com/spf13/cobra.(*Command).Execute(...)
	/home/knakayam/.go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:800
main.main()
	/home/knakayam/.go/src/github.com/knative/client/cmd/kn/main.go:29 +0x45

Actual result

• Please refer to step-2

Proposal patch

#73