This fails to compile on line 18, but it should work because the field is final so the null check ensures the field can't be null

dart-null-safe-final-field-fail.dart

// Welcome to the null safety version of DartPad!
// This site has a dev channel release of the
// Dart SDK with null safety enabled. You can
// play around with code of your own, or pick a
// learning exercise from the snippets menu at
// the top-right of this page.

void main() {

}

class Hello {
    final String? stackTraceString;
  
    Hello({this.stackTraceString});

    StackTrace? get stacktrace =>
      stackTraceString != null ? StackTrace.fromString(stackTraceString) : null;

}