List of steps to reproduce

• Create a .map in the Wikimedia Commons "Data" namespace.
• Paste this minimal example:

{
    "license": "CC0-1.0",
    "zoom": 11,
    "latitude": 40.782222,
    "longitude": -73.965278,
    "data": {
        "type": "Feature",
        "feature": {
			"type": "ExternalData",
			"service": "geoshape",
			"query": "select ?id from foobar"
		}
    }
}

• Preview the map.

What happens?:
An error appears:

<mapframe>: The JSON content is not valid GeoJSON+simplestyle. The first validation error is on the element "/0/query": "The property query is required".

What should have happened instead?:
The error should be something like "ExternalData not supported here"—or the query should execute (currently not supported, see T155927). There is no missing "query" property, I believe this is reported by the schema rule here, but "/0" is confusing.

No error is reported if the externalData is replaced by a geometry feature.

Software version (if not a Wikimedia wiki), browser information, screenshots, other information, etc.: