"Try online" button does not work

Steps to reproduce:
1. go to https://vega.github.io/vega-lite/
2. click "try online"
3. see `Invalid specification {"$schema":"https://vega.github.io/schema/vega-lite/v5.json"}. Make sure the specification includes at least one of the following properties: "mark", "layer", "facet", "hconcat", "vconcat", "concat", or "repeat".`

**Vega-Editor 2dd3a8515b08399c340c8604ba5a71a0341ebd36**