check unit_of_measurements for being None

Author: uwzm

frost_sta_client/model/multi_datastream.py

@@ -299,7 +299,7 @@ def __getstate__(self):
             data['Sensor'] = self.sensor
         if self.properties is not None and self.properties != {}:
             data['properties'] = self.properties
-        if len(self.unit_of_measurements) > 0:
+        if self.unit_of_measurements is not None and len(self.unit_of_measurements) > 0:
             data['unitOfMeasurements'] = self.unit_of_measurements
         if len(self.multi_observation_data_types) > 0:
             data['multiObservationDataTypes'] = self.multi_observation_data_types