CallbackProperty causes crash for polygon entity on terrain

[hpinkos]

For the below code example, the polygon only works if you set height: 0
Otherwise it crashes here because groundPrimitives is undefined

var viewer = new Cesium.Viewer('cesiumContainer');
viewer.entities.add({
    polygon : {
        //height: 0,
        hierarchy : new Cesium.CallbackProperty(function() { 
            return Cesium.Cartesian3.fromDegreesArray([-115.0, 37.0,
                                                        -115.0, 32.0,
                                                        -107.0, 33.0,
                                                        -102.0, 31.0,
                                                       -102.0, 35.0]);
                                                      }, false),
        material : Cesium.Color.RED
    }
});