Check scheduler exists before checking if it has stopped - a cause of test flakiness

Author: cianc

codecarbon/emissions_tracker.py

@@ -851,7 +851,11 @@ def _measure_power_and_energy(self) -> None:
             raise e
 
         warning_duration = self._measure_power_secs * 3
-        if last_duration > warning_duration and not self._scheduler._stopped:
+        if (
+            last_duration > warning_duration
+            and self._scheduler
+            and not self._scheduler._stopped
+        ):
             warn_msg = (
                 "Background scheduler didn't run for a long period"
                 + " (%ds), results might be inaccurate"