How to handle gpx in cesium?

This example does not work, it does not interact with gpx:
https://notizen.netzjahre.com/cesium3d/vogelsang-2016-10-01-gpx/index.html

The code looks like this:
https://notizen.netzjahre.com/cesium3d/vogelsang-2016-10-01-gpx/cesium-gpx-malfunction.txt

What is wrong?
Thx in advance for your hints.
Josef “cverwalter”