whenOrNull<TResult extends Object?>  method 
- @optionalTypeArgs
- TResult? $default(- PaymentElementBillingDetails? billingDetails,
- PaymentElementPaymentMethodDefaults? paymentMethods
 
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( PaymentElementBillingDetails? billingDetails,  PaymentElementPaymentMethodDefaults? paymentMethods)?  $default,) {final _that = this;
switch (_that) {
case _PaymentElementDefaultValues() when $default != null:
return $default(_that.billingDetails,_that.paymentMethods);case _:
  return null;
}
}