Update

app.py

@@ -586,12 +586,14 @@ def preprocess_image_end(image_end):
 
 
 def add_drag(tracking_points):
-    tracking_points.append([])
+    if not tracking_points or tracking_points[-1]:
+        tracking_points.append([])
     return tracking_points
 
 
 def delete_last_drag(tracking_points, first_frame_path, last_frame_path):
-    tracking_points.pop()
+    if tracking_points:
+        tracking_points.pop()
     transparent_background = Image.open(first_frame_path).convert("RGBA")
     transparent_background_end = Image.open(last_frame_path).convert("RGBA")
     w, h = transparent_background.size
@@ -633,13 +635,16 @@ def delete_last_drag(tracking_points, first_frame_path, last_frame_path):
 
 
 def delete_last_step(tracking_points, first_frame_path, last_frame_path):
-    tracking_points[-1].pop()
+    if tracking_points and tracking_points[-1]:
+        tracking_points[-1].pop()
     transparent_background = Image.open(first_frame_path).convert("RGBA")
     transparent_background_end = Image.open(last_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]