Variable command returning null does not cancel debug session launch

[weinand]

• VSCode Version: 1.9.0
• OS Version: macOS

Steps to Reproduce:

1. use this launch config:

{
	"type": "node",
	"request": "attach",
	"name": "Attach to Port",
	"port": 5858,
	"processId": "${command.PickProcess}"
}

2. start "Attach to Port"
3. Press 'Esc' to cancel

Observe:
this error appears:

I've verified that the variable command returns a null in this case but this only cancels the variable substitution (and leaves the variable expression intact) but not the debug session launch.