DragDrop Mixin behaves differently in SandCastle than in regular Cesium

Hello, I am trying to make an app that uses Drag and Drop. Noticing different behaviour in SandCastle then when running against a regular application. Here is the code:

var viewer = new Cesium.Viewer(‘cesiumContainer’);

viewer.selectedEntityChanged.addEventListener(function () {

I drop a .geojson file onto the globe in SandCastle and click on one of the points. The entity is shown in the log as expected.
However when I run the exact same code without sandcastle the log shows that selectedEntity is undefined. Why is the behaviour different between SandCastle on regular Cesium?