Dart analysis gives null error even if I had performed a null check

I have performed a null check but still I'm getting the below error.

error: The argument type 'String?' can't be assigned to the parameter type 'String'. (argument_type_not_assignable at [sitewalk] lib/modules/floor_plan/widgets/dialog_nested/dialog_nested.dart:122)

Screenshot below for better understanding:

Now if I use the null assertion operator(!) the error will be gone. But I wanted to know why it's giving an error on the first place?

Is it because if we use multi thread this value can be changed which might cause an error?

The reason is actually that subclasses of your Test class may override the field with a getter that doesn't consistently give the same value back. Like this:

class Test2 extends Test {
  const Test2({super.key, super.errormessage});

  @override
  String? get errorMessage => Random().nextBool() ? null : 'aaa';
}

In this case the null check might pass the first time but when getting it for the Text it might return null

You need a variable, that is considered to be a candidate for type promotion from String? to String. The easiest way is to have a local variable:

@override
Widget build(BuildContext context) {
  final localErrorMessage = errorMessage;
  if(localErrorMessage == null) {
    return SizedBox.shrink();
  } else {
    return Text(localErrorMessage);
  }
}

use this code

  class MyClass extends StatelessWidget {
  final String? errorMessage;
  const MyClass({Key? key, this.errorMessage,}) : super(key: key);
    @override
     Widget build(BuildContext context) {
      if (errorMessage == null) {
      return const SizedBox.shrink();
    } else {
      return Text("$errorMessage");
    }
  }
}

have good time;

You can do it like:

Put bang operator to end of the 'errorMessage'

class Test extends StatelessWidget {
  final String? errorMessage;

  const Test({
    Key? key,
    this.errorMessage,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    if (errorMessage == null) {
      return const SizedBox.shrink();
    } else {
      return Text(errorMessage!);
    }
  }
}