Hi @seb90 I also tested this out and I found that I got that None response, too, so I'm with you! Coincidentally, I was involved in the original development of this operation, and I know it did work, once upon a time 🦄🌈👸🏼.
Meraki Support is your best bet for a solution. But I know that traffic analytics are calculated according to one of two frameworks: the first, TA, is legacy. The newer one, NBAR, is what I'm using today and have been using for months. It's possible that the API operation queries TA data, finds none, then response None. Probably not an ideal result. Coincidentally, my Organization > Summary Report page "Top applications by usage" does show results, so perhaps the GUI page is using a different operation in the backend.
Either way, I think the network ID filter is a red herring, and I recommend reporting it to Meraki Support.