SpreadsheetPrinter exports with "Notice: Undefined variable: values"

[mwjames]

Setup

{
    "SMWElasticStore": {
        "mysql": "10.1.40-MariaDB-0ubuntu0.18.04.1",
        "es": "6.5.4"
    },
    "smw": "3.1.0-alpha",
    "mediawiki": "1.33.0",
    "php": "7.2.19-0ubuntu0.18.04.1"
}

Issue

The format=spreadsheet export produces:

Notice: Undefined variable: values in /var/www/html/sbxsmw/w/extensions/SemanticResultFormats/formats/spreadsheet/SpreadsheetPrinter.php on line 316
--
 
Warning: join(): Invalid arguments passed in /var/www/html/sbxsmw/w/extensions/SemanticResultFormats/formats/spreadsheet/SpreadsheetPrinter.php on line 316

Steps to reproduce

https://sandbox.semantic-mediawiki.org/w/index.php?title=SpreadsheetTest

{{#ask: [[Has number::+]]
 |?Has number
 |?Has text
 |format=spreadsheet
}}

[kghbln]

@s7eph4n It will be great if you could have a peep.