Resolve "release v1.4.0"

Closes #317 (closed)

mlair/run_modules/post_processing.py

@@ -680,9 +680,10 @@ class PostProcessing(RunEnvironment):
             external_data = self._get_external_data(station, path)  # test data
 
             # test errors
-            errors[station] = statistics.calculate_error_metrics(*map(lambda x: external_data.sel(type=x),
-                                                                      [self.forecast_indicator, "obs"]),
-                                                                 dim="index")
+            if external_data is not None:
+                errors[station] = statistics.calculate_error_metrics(*map(lambda x: external_data.sel(type=x),
+                                                                          [self.forecast_indicator, "obs"]),
+                                                                     dim="index")
             # skill score
             competitor = self.load_competitors(station)
             combined = self._combine_forecasts(external_data, competitor, dim="type")