var rasters = document.getItems({
    type: Raster,
    selected: true
});

if(rasters.length) {
    var raster = rasters[0];
    var color = raster.getAverageColor(raster.bounds);
    print(color); // null

    var path = new Path.Rectangle(raster.bounds);
    path.fillColor = rasters.first.getAverageColor(path);
    print(path.fillColor); // null

    path.remove();
}