How to use data from Provider during initState in Flutter application

You need to use the context to call Provider.of() so you can add addPostFrameCallback() which is called after the first build, there you can use the context

@override
void initState() {
    super.initState();

    WidgetsBinding.instance.addPostFrameCallback((timeStamp) {
      auth = Provider.of<Auth>(context, listen: false);
    });
}