whenOrNull<TResult extends Object?>  method 
- @optionalTypeArgs
 
- TResult? relationship(
- Relationship data
 
 - TResult? notFoundActor(
- NotFoundActor data
 
 - TResult? unknown()?,
 
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( Relationship data)?  relationship,TResult? Function( NotFoundActor data)?  notFoundActor,TResult? Function( Map<String, dynamic> data)?  unknown,}) {final _that = this;
switch (_that) {
case UGraphGetRelationshipsRelationshipsRelationship() when relationship != null:
return relationship(_that.data);case UGraphGetRelationshipsRelationshipsNotFoundActor() when notFoundActor != null:
return notFoundActor(_that.data);case UGraphGetRelationshipsRelationshipsUnknown() when unknown != null:
return unknown(_that.data);case _:
  return null;
}
}