[Designer] The payload editor warns of missing version property in nested cards

[sabberworm]

Problem Description

Using the activity update sample from https://adaptivecards.io/samples/ActivityUpdate.html, the designer shows an error in the JSON:

However, the spec states that version is optional for cards nested within Action.ShowCard. I suppose this is a side-effect of using the JSON schema to validate the code in the editor. The fact that an official sample (the first one to be listed at that) shows up as invalid in the official editor seems rather unfortunate.

Card JSON

{
    "type": "AdaptiveCard",
    "actions": [
        {
            "type": "Action.ShowCard",
            "title": "Set due date",
            "card": {
                "type": "AdaptiveCard",
                "actions": [
                    {
                        "type": "Action.Submit",
                        "title": "OK"
                    }
                ]
            }
        }
    ],
    "$schema": "http://adaptivecards.io/schemas/adaptive-card.json",
    "version": "1.6"
}