Add error handling

app.py

@@ -498,7 +498,8 @@ def delete_last_drag(tracking_points, first_frame_path, drag_mode):
         color = (255, 0, 0, 255)
     elif drag_mode == "camera":
         color = (0, 0, 255, 255)
-    tracking_points.pop()
+    if tracking_points:
+        tracking_points.pop()
     transparent_background = Image.open(first_frame_path).convert("RGBA")
     w, h = transparent_background.size
     transparent_layer = np.zeros((h, w, 4))