[BUG] [dart] How to correctly define an object reference that can be null in the generated code

Description

Generated code contains

  ///
  /// Please note: This property should have been non-nullable! Since the specification file
  /// does not include a default value (using the "default:" property), however, the generated
  /// source code must fall back to having a nullable type.
  /// Consider adding a "default:" property in the specification file to hide this note.
  ///
  UrlPreview? urlPreview;

which actually should be nullable. But I couldn't find a way how to define a property with a reference schema that can be null, so that this note disappears

this is the json spec

      "PostMetaData": {
        "title": "PostMetaData",
        "x-stoplight": {
          "id": "wjwait32lwf1r"
        },
        "type": "object",
        "properties": {
          "url_preview": {
            "$ref": "#/components/schemas/UrlPreview",
          }
        }
      }

how do I correctly specify a property that is optional and should be nullable in the generated Dart code?

Start with the generated Dart property and the PostMetaData schema shown in the issue, then trace how the generator handles nullable properties that use $ref. Verify the OpenAPI specification syntax and generated output. Done means an optional UrlPreview reference is generated as nullable without the explanatory warning.