Add billboard crash with globe :false option

[zwzhlj100000]

Add billboard crash without globe.

var viewer = new Cesium.Viewer('cesiumContainer',{globe:false});
viewer.entities.add({
position : Cesium.Cartesian3.fromDegrees(-75.59777, 40.03883),
billboard :{
image : '../images/Cesium_Logo_overlay.png'
}
});

[ggetz]

Thanks for the report @zwzhlj100000, addressed in #6110