Do not allow push configuration to be null

[FrNecas]

It would throw an exception.

[ondryaso]

And what exactly happens when you do this? Doesn't it throw an exception when you call the notifications endpoint and attempt to make a subscription? I would expect it to. If it does, you've just pushed the problem around to a different place.

I don't think it's wrong to throw an exception during startup when the app is misconfigured. Certainly better than throwing them in the endpoints.