Added assert and error message

point_tracker/tracker2d.py

@@ -79,9 +79,7 @@ class Tracker2D:
 
         """
 
-        if len(self.images) is 0:
-            print("WARN: you need to load images first")
-            return
+        assert len(self.images) != 0, "Images need to be loaded first"
 
         [self.back_subtractor.apply(image, learningRate=-1) for image in self.images[:-6]]