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( String $type,  int seq,  String did,  DateTime time,  bool active, @AccountStatusConverter()  AccountStatus? status,  Map<String, dynamic>? $unknown)?  $default,) {final _that = this;
switch (_that) {
case _Account() when $default != null:
return $default(_that.$type,_that.seq,_that.did,_that.time,_that.active,_that.status,_that.$unknown);case _:
  return null;
}
}