Add safety check for empty end_effector data

app.py

@@ -60,7 +60,19 @@ def prepare_data():
     right_pos = {'x': [], 'y': [], 'z': []}
     right_rot = {'yaw': [], 'pitch': [], 'roll': []}
     
-    frame_keys = sorted([int(k) for k in end_effector.keys() if k.isdigit()])
+    if not end_effector:
+        return {
+            'times': times,
+            'left_pos': left_pos,
+            'left_rot': left_rot,
+            'right_pos': right_pos,
+            'right_rot': right_rot
+        }
+    
+    try:
+        frame_keys = sorted([int(k) for k in end_effector.keys() if str(k).isdigit()])
+    except:
+        frame_keys = []
     
     for frame_idx in frame_keys:
         frame_key = str(frame_idx)