Update docs for pybind11::custom_type_setup

Author: XuehaiPan

docs/advanced/classes.rst

@@ -1384,13 +1384,21 @@ You can do that using ``py::custom_type_setup``:
            auto *type = &heap_type->ht_type;
            type->tp_flags |= Py_TPFLAGS_HAVE_GC;
            type->tp_traverse = [](PyObject *self_base, visitproc visit, void *arg) {
-               auto &self = py::cast<OwnsPythonObjects&>(py::handle(self_base));
-               Py_VISIT(self.value.ptr());
+               // https://docs.python.org/3/c-api/typeobj.html#c.PyTypeObject.tp_traverse
+   #if PY_VERSION_HEX >= 0x03090000  // Python 3.9
+               Py_VISIT(Py_TYPE(self_base));
+   #endif
+               if (py::detail::is_holder_constructed(self_base)) {
+                   auto &self = py::cast<OwnsPythonObjects&>(py::handle(self_base));
+                   Py_VISIT(self.value.ptr());
+               }
                return 0;
            };
            type->tp_clear = [](PyObject *self_base) {
-               auto &self = py::cast<OwnsPythonObjects&>(py::handle(self_base));
-               self.value = py::none();
+               if (py::detail::is_holder_constructed(self_base)) {
+                   auto &self = py::cast<OwnsPythonObjects&>(py::handle(self_base));
+                   self.value = py::none();
+               }
                return 0;
            };
        }));