Edge case in https://w3c.github.io/ServiceWorker/#clients-openwindow

https://w3c.github.io/ServiceWorker/#clients-openwindow step 7 .1 reads as:
Let newContext be a new top-level browsing context.

In some edge cases, newContext might be null, and it is not clear what should happen.
With the current spec wording, it seems openWindow promise would never resolve, which is not great.

I see two possibilities:

1. Resolve the promise with null. This somehow aligns with window.open which returns null in that case. The web app will not be able to differentiate the case where a tab is opened vs. a tab is not opened
2. Reject the promise. This allows the web app to differentiate the case where a tab is opened vs. no tab is opened. That leaves a chance for the web app to do alternative things (focus an existing tab if any and navigate it for instance).

Thoughts?

Start by reading the Clients.openWindow algorithm at Service Worker specification step 7.1 and the issue's comparison with window.open. Determine the intended result when newContext is null, then update the specification wording so the promise behavior is explicit. Done means the edge case has an agreed resolution and the algorithm no longer leaves the promise outcome ambiguous.