Google maps traffic card: oops something went wrong with JavaScript

Hi,

I have managed to get a iframe card on the screen. I did it by following these steps:

First you have to sign up for their developer program. It’s free if you’re not hammering their server.
https://console.cloud.google.com/google/maps-apis/overview?

Then you have to create some html that you can embed into the card:

Save this to your www directory.

Then create an iframe Lovelace card:
url: /local/traffic.html
aspect_ratio: 100%
type: iframe

When I open the view I see for a short moment the desired card with traffic info. But then it dissapeares and I get a message: Oops, something went wrong. This page didn’t load Google Maps correctly. See the JavaScript console for technical details.

Someone knows what to do? How can I open the JavaScript console for further investigation?

Did you ever get this working?