Issue With dart null safety 1

fields.dart

class A {
  A(this.text);

  final String? text; // Final field
}

class B implements A {
  bool _first = true;

  @override
  // Not final anymore
  String? get text {
    if (_first) {
      _first = false;
      // On the first call return null
      return 'Some string';
    } else {
      // On the next calls return null
      return null;
    }
  }
}

void main() {
  final A example = B();
  print(example.text); // Prints 'Some string'
  print(example.text); // Prints null
}

fields_error.dart

void main() {
  final A example = B();
  if (example.text != null) {
    print(example.text!.length);
  }
}

main.dart

final String? text;

@override
Widget build(BuildContext context) {
  return Column(
    children: [
      if (text != null) Text(text!), // Won't compile without `!` in `text!` 
    ],
  );
}

main_safe.dart

final String? text;

@override
Widget build(BuildContext context) {
  final _text = text;
  return Column(
    children: [
      if (_text != null) Text(_text), // The `!` is not longer needed
    ],
  );
}