Unable to view 3D data of Polygon

[SurajitChowbey]

Please help me, getting a black screen after enabling the 3D mode: My code is as follows:

` var ol2d = new ol.Map({ layers: [ new ol.layer.Tile({ source: new ol.source.OSM() }), new ol.layer.Vector({
/altitudeMode: 'clampToGround',/ // to drape polygon on 3d title: 'test', visible: true, source: new ol.source.Vector({ url: 'data/geojson_data_new_WGS84.geojson', projection: 'EPSG:4326', format: new ol.format.GeoJSON() }) }) ], controls: ol.control.defaults({ attributionOptions: /** @type {olx.control.AttributionOptions} */ ({ collapsible: false }) }), target: 'map2d', view: new ol.View({ center: ol.proj.transform([25, 20], 'EPSG:4326', 'EPSG:3857'), zoom: 3 }) });

var ol3d = new olcs.OLCesium({ map: ol2d
});

`

Demo GeoJson data is attached. geojson_data_new_WGS84.geojson.txt](https://github.com/openlayers/ol-cesium/files/1004358/geojson_data_new_WGS84.geojson.txt)