FFAppState issue

I have created a function which will generate alpha numeric characters. I have also created a APPSTATE which I am trying to update in my function, but when I am add the line in it, it throws error.

I am getting this error:
The function 'FFAppState' isn't defined.
Try importing the library that defines 'FFAppState', correcting the name to the name of an existing function, or defining a function named 'FFAppState'.

FFAppState().appliRefNo = result;

String? applicationRefNum() {
  /// MODIFY CODE ONLY BELOW THIS LINE

  String chars =
      'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';
  String result = '';
  final random = math.Random();
  for (int i = 0; i < 5; i++) {
    result += chars[random.nextInt(chars.length)];
  }

// this is the to update appstate
  FFAppState().appliRefNo = result;

  return result;

  /// MODIFY CODE ONLY ABOVE THIS LINE
}