nil pointer dereference when using DAG target

[jessesuen]

Is this a BUG REPORT or FEATURE REQUEST?:

When using the target feature of a DAG template, the following panic is encountered:

time="2018-05-24T09:45:18Z" level=info msg="Updating node dag-diamond-lz84l.C (dag-diamond-lz84l-445793586) status Running -> Succeeded"
time="2018-05-24T09:45:18Z" level=info msg="Updated phase Running -> Error" namespace=default workflow=dag-diamond-lz84l
time="2018-05-24T09:45:18Z" level=info msg="Updated message  -> runtime error: invalid memory address or nil pointer dereference" namespace=default workflow=dag-diamond-lz84l
time="2018-05-24T09:45:18Z" level=info msg="Marking workflow completed" namespace=default workflow=dag-diamond-lz84l
time="2018-05-24T09:45:18Z" level=error msg="Recovered from panic: runtime error: invalid memory address or nil pointer dereference
goroutine 66 [running]:
runtime/debug.Stack(0xc4208e6120, 0x1889680, 0x18e7370)
	/usr/local/go/src/runtime/debug/stack.go:24 +0xa7
github.com/argoproj/argo/workflow/controller.(*wfOperationCtx).operate.func1(0xc4208e6120)
	/root/go/src/github.com/argoproj/argo/workflow/controller/operator.go:108 +0xb5
panic(0x10422c0, 0x18e7370)
	/usr/local/go/src/runtime/panic.go:491 +0x283
github.com/argoproj/argo/workflow/controller.(*wfOperationCtx).processNodeOutputs(0xc4208e6120, 0xc42055f1c0, 0xc420113a64, 0x7, 0x0)
	/root/go/src/github.com/argoproj/argo/workflow/controller/operator.go:1164 +0x5d
github.com/argoproj/argo/workflow/controller.(*wfOperationCtx).executeDAG(0xc4208e6120, 0xc4207d67a0, 0x11, 0xc4203cc780, 0x0, 0x0, 0x0)
	/root/go/src/github.com/argoproj/argo/workflow/controller/dag.go:152 +0x5e1
github.com/argoproj/argo/workflow/controller.(*wfOperationCtx).executeTemplate(0xc4208e6120, 0xc42029e770, 0x7, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc4207d67a0, ...)
	/root/go/src/github.com/argoproj/argo/workflow/controller/operator.go:870 +0x9ca
github.com/argoproj/argo/workflow/controller.(*wfOperationCtx).operate(0xc4208e6120)
	/root/go/src/github.com/argoproj/argo/workflow/controller/operator.go:146 +0x37c
github.com/argoproj/argo/workflow/controller.(*WorkflowController).processNextItem(0xc420408000, 0x82e400)
	/root/go/src/github.com/argoproj/argo/workflow/controller/controller.go:216 +0x27c
github.com/argoproj/argo/workflow/controller.(*WorkflowController).runWorker(0xc420408000)
	/root/go/src/github.com/argoproj/argo/workflow/controller/controller.go:182 +0x2b
github.com/argoproj/argo/workflow/controller.(*WorkflowController).(github.com/argoproj/argo/workflow/controller.runWorker)-fm()
	/root/go/src/github.com/argoproj/argo/workflow/controller/controller.go:149 +0x2a
github.com/argoproj/argo/vendor/k8s.io/apimachinery/pkg/util/wait.JitterUntil.func1(0xc420264130)
	/root/go/src/github.com/argoproj/argo/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:133 +0x5e
github.com/argoproj/argo/vendor/k8s.io/apimachinery/pkg/util/wait.JitterUntil(0xc420264130, 0x3b9aca00, 0x0, 0x1, 0xc4200661e0)
	/root/go/src/github.com/argoproj/argo/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:134 +0xbd
github.com/argoproj/argo/vendor/k8s.io/apimachinery/pkg/util/wait.Until(0xc420264130, 0x3b9aca00, 0xc4200661e0)
	/root/go/src/github.com/argoproj/argo/vendor/k8s.io/apimachinery/pkg/util/wait/wait.go:88 +0x4d
created by github.com/argoproj/argo/workflow/controller.(*WorkflowController).Run
	/root/go/src/github.com/argoproj/argo/workflow/controller/controller.go:149 +0x597
" namespace=default workflow=dag-diamond-lz84l

How to reproduce it (as minimally and precisely as possible):

add target: C to the example/dag-diamond.yaml template:

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
  generateName: dag-diamond-
spec:
  entrypoint: diamond
  templates:
  - name: echo
    inputs:
      parameters:
      - name: message
    container:
      image: alpine:3.7
      command: [echo, "{{inputs.parameters.message}}"]
  - name: diamond
    dag:
      target: C     # <<<<<<<<<<< add this
      tasks:
      - name: A
        template: echo
        arguments:
          parameters: [{name: message, value: A}]
      - name: B
        dependencies: [A]
        template: echo
        arguments:
          parameters: [{name: message, value: B}]
      - name: C
        dependencies: [A]
        template: echo
        arguments:
          parameters: [{name: message, value: C}]
      - name: D
        dependencies: [B, C]
        template: echo
        arguments:
          parameters: [{name: message, value: D}]

Environment:

• Argo version: 2.1.0