code: def test_has_ref(self) -> None:
      prop0 = bcpn.Nullable(Int)
      assert not prop0.has_ref
      prop1 = bcpn.Nullable(Instance(_TestModel))
      assert prop1.has_ref
  
imports: from __future__ import annotations
  import pytest
  import warnings
  from typing import TYPE_CHECKING, Union
  from bokeh.core.properties import Dict, Instance, Int, List, String
  from bokeh.core.property.wrappers import PropertyValueDict, PropertyValueList
  from bokeh.util.warnings import BokehDeprecationWarning
  from tests.support.util.api import verify_all
  from _util_property import _TestHasProps, _TestModel
  import bokeh.core.property.nullable
label_desc: Use image node_id 6 for calling the Test_Nullable obj's underlying member method code with example usage: obj.test_has_ref() without return types