Snipped for is null problem

main.dart

extension Nulls on dynamic {
  bool get isNull => this == null;
}

class Foo {
  String getA(){
    return null;
  }
}

void main() {
  var foo = Foo();
  var bar = "";
  print(foo.getA().isNull);
  print(bar.isNull);
  
  bar = null;
  print(bar.isNull);
}