Update inference.py

inference.py

@@ -361,7 +361,7 @@ def do_associative_tracking(inference, image_shapes, width, height, image_meter_
             low_boxes = low_preds[key]
             high_boxes = high_preds[key]
             boxes = (low_boxes, high_boxes)
-            if len(low_boxes) + len(high_boxes) > 0:
+            if low_boxes is not None and high_boxes is not None:
                 tracker.update(boxes)
             else:
                 print("empty")