addError method Null safety
- @protected
 - @mustCallSuper
 - @override
 
- Object error,
 - [StackTrace? stackTrace]
 
@mustCallSuper, @protected, inherited
    Reports an error which triggers onError with an optional StackTrace.
Implementation
@protected
@mustCallSuper
@override
void addError(Object error, [StackTrace? stackTrace]) {
  onError(error, stackTrace ?? StackTrace.current);
}