$ az provider register --namespace "Microsoft.Storage"
Registering is still on-going. You can monitor using 'az provider show -n Microsoft.Storage'
null

"You can monitor using az provider show..." doesn't seem like a reasonable way to handle a request for registration. If the rest of the script requires that resource provider to be registered, the consumer of this command will need to loop poling az provider show to determine the request result.

There should be wait / no-wait on this command. Wait should wait for the registration to complete and return either an error or the body of the provider.