Add error handling

app.py

@@ -536,11 +536,14 @@ def delete_last_step(tracking_points, first_frame_path, drag_mode):
         color = (255, 0, 0, 255)
     elif drag_mode == "camera":
         color = (0, 0, 255, 255)
-    tracking_points[-1].pop()
+    if tracking_points and tracking_points[-1]:
+        tracking_points[-1].pop()
     transparent_background = Image.open(first_frame_path).convert("RGBA")
     w, h = transparent_background.size
     transparent_layer = np.zeros((h, w, 4))
     for track in tracking_points:
+        if not track:
+            continue
         if len(track) > 1:
             for i in range(len(track) - 1):
                 start_point = track[i]