I think you can simplify the logic here by eliminating the `if` and just doing:

`viewerElement.insertBefore(selectionIndicatorContainer, viewerElement.firstChild.nextSibling);`

If you want to be more futureproof, perhaps instead of `firstChild`, `viewerElement.querySelector('.cesium-viewer-cesiumWidgetContainer')`.

Also, it turns out that `viewerElement.children.length > 1` is always true.  Viewer always adds the cesiumWidget container  and a toolbar, no matter what is turned off.