Run custom Cesium function only after viewer.zoomTo() has been executed - promise

Question

I have a CesiumJS function that I want to be executed after and only after having performed a zoomTo(), which makes me sure the terrain has been fully loaded (as stated in the doc)!
It's because this function should actually compute the intersection between a ray and the terrain, so the terrain has to be fully loaded for its execution to be successful.
In the current state, the following code returns: null super quickly (it doesn't wait at all for the terrain):
e.g.:
var viewer = new Cesium.Viewer('cesiumContainer', {
imageryProvider : new Cesium.BingMapsImageryProvider({
url : 'https://dev.virtualearth.net',
key : $(BING_KEY),
mapStyle : Cesium.BingMapsStyle.AERIAL
}),
terrainProvider: new Cesium.CesiumTerrainProvider({
url : 'https://api.maptiler.com/tiles/terrain-quantized-mesh/?key$(MKEY)'
});
baseLayerPicker : false,
infoBox : false,
});
var p0 = [10.7465, 48.3147, 1500];
var p1 = [10.7475, 48.3127, 1000];
var start = new Cesium.Cartesian3.fromDegrees(...p0);
var end = new Cesium.Cartesian3.fromDegrees(...p1);
var entity = viewer.entities.add({
polyline : {
positions : Cesium.Cartesian3.fromDegreesArrayHeights([
...(p0.concat(p1))
]),
width : 4,
material : new Cesium.PolylineGlowMaterialProperty({
glowPower : 1,
color : Cesium.Color.GREEN
})
}
});
viewer.zoomTo(viewer.entities);
// then run this function:
function pickGlobeIntersection(viewer, p0, p1) {
var direction = new Cesium.Cartesian3();
Cesium.Cartesian3.subtract(p1, p0, direction);
console.log("direction: ", direction);
Cesium.Cartesian3.normalize(direction, direction);
var ray = new Cesium.Ray(p0, direction);
var hitPos = viewer.scene.globe.pick(ray, viewer.scene);
if ((hitPos !== undefined) && (hitPos !== null)) {
console.log("hitPos: ", hitPos);
return hitPos;
} else {
console.log("hitPos is null!");
return null;
}
}
pickGlobeIntersection(viewer, start, end);
(replace the viewer by the default one if code is used in the sandcastle)
How can I make sure the function is executed only after the terrain has been fully loaded?