dehaze | Project for course Digit Image Process

Your problem is that initState is not async (and it can't be async), so your getData will likely (or surely) be completed later than the build method is called. Since you set displayName in getData, and you marked it as late, you will get this error. That's why it is working after hot reload, because at that time displayName is already initalized.

One possible solution is to call SharedPreferences.getInstance() (this is why your getData needs to be async) somewhere earlier in your application, for example in main() async function, store the result of getInstance using a ChangeNotifierProvider (or any other way that allows you to use this value in this widget), and use its value in getData. This way getData does not need to be async, and displayName will be initalized before build method is called. This can be useful if you use SharedPreferences in more than one place, because you don't have to await the result of getInstance each time.

Another approach is not declaring displayName as late, but allow it to be null, and then in the build method you can use a FutureBuilder which completes upon getData is executed, and the building of Scaffold will only be started after this future completes. In this case you can display a progress indicator while the future is not completed.