whenOrNull<TResult extends Object?>  method 
- @optionalTypeArgs
- TResult? $default(- Element card,
- BillingDetails? billingDetails
 
- TResult? id(- String id
 
- TResult? token(- CardTokenPaymentMethod card,
- BillingDetails? billingDetails
 
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(@ElementConverter()  Element card, @JsonKey(name: "billing_details")  BillingDetails? billingDetails)?  $default,{TResult? Function( String id)?  id,TResult? Function( CardTokenPaymentMethod card, @JsonKey(name: "billing_details")  BillingDetails? billingDetails)?  token,}) {final _that = this;
switch (_that) {
case _CardPaymentMethodRef() when id != null:
return id(_that.id);case _CardPaymentMethodDefault() when $default != null:
return $default(_that.card,_that.billingDetails);case _CardPaymentMethodDetailsToken() when token != null:
return token(_that.card,_that.billingDetails);case _:
  return null;
}
}