fix typing

Author: wedamija

tests/sentry/monitors/test_utils.py

@@ -28,6 +28,7 @@ def test_creates_data_source_and_detector_for_new_monitor(self):
     def test_idempotent_for_existing_data_source(self):
         ensure_cron_detector(self.monitor)
         detector = get_detector_for_monitor(self.monitor)
+        assert detector
         ensure_cron_detector(self.monitor)
         detector_after = get_detector_for_monitor(self.monitor)
         assert detector.id == detector_after.id