UPDATE: This should be resolved shortly (as in today 3/2/2017)
You are creating a Mobile App or Azure App Services and setting up the push notifications for the first time
Calling Register or RegisterAsync in the client code can result in a 400 status code coming back with an inner exception of: 'Cannot read property 'listRegistrationsByTag' of undefined'
The Portal blade does not set the Application Setting MS_NotificationHubName
Add MS_NotificationHubName in the Application settings. The name will be the name of your Notification Hub. It is the last item after the \ in the setting MS_NotificationHubId or you can get in from the notification Hub blade:
Once this is fixed the Portal Blade will set the MS_NotificationHubName for you when you walk through the Portal setup of Push notifications. At that time I will update this Blog! If you need further help, you can certainly open a support case through the Azure Portal.