fix: memoized decorator memory leak (#23139)

(cherry picked from commit 79274eb5bc)

tests/integration_tests/utils_tests.py

@@ -991,6 +991,7 @@ class TestUtils(SupersetTestCase):
         slc = self.get_slice("Girls", db.session)
         dashboard_id = 1
 
+        assert slc.viz is not None
         resp = self.get_json_resp(
             f"/superset/explore_json/{slc.datasource_type}/{slc.datasource_id}/"
             + f'?form_data={{"slice_id": {slc.id}}}&dashboard_id={dashboard_id}',