Cesium 控制地形TerrainProvider 显示与移除

function toggleTerrain(){    
    var scene = viewer.scene;
    var terrainProvider;
    if (scene.terrainProvider._url){
        //add a simple terain so no terrain shall be preseneted
        terrainProvider = new Cesium.EllipsoidTerrainProvider({});
    }
    else {
        //enable the terain
        terrainProvider = new Cesium.CesiumTerrainProvider({
            url : '//cesiumjs.org/stk-terrain/tilesets/world/tiles',
            requestWaterMask: true
        });
    }
    scene.terrainProvider = terrainProvider;
}