fix bug concerning input field

horizon-metrics.py

@@ -176,6 +176,10 @@ class HorizonMetrics(evaluate.Metric):
             float: The computed horizon error.
         """
 
+        if (self.ground_truth_det is None or self.predictions is None):
+            self.ground_truth_det = references
+            self.predictions = predictions
+
         # calculate erros and store values in slope_error_list and midpoint_error_list
         for annotated_horizon, proposed_horizon in zip(self.ground_truth_det,
                                                        self.predictions):