whenOrNull<TResult extends Object?>  method 
- @optionalTypeArgs
A variant of when that fallback to returning null
It is equivalent to doing:
switch (sealedClass) {
  case Subclass(:final field):
    return ...;
  case _:
    return null;
}
Implementation
@optionalTypeArgs TResult? whenOrNull<TResult extends Object?>(TResult? Function( List<Exchange> exchanges,  int total)?  $default,) {final _that = this;
switch (_that) {
case _GetExchangesResult() when $default != null:
return $default(_that.exchanges,_that.total);case _:
  return null;
}
}