fix: small defense against elements that might have no pose in history

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: DamienGilliard

src/gh/diffCheck/diffCheck/df_error_estimation.py

@@ -285,7 +285,8 @@ def add_history(self, pose_history : dict[str, DFPosesBeam]):
         """
         self.pose_history = pose_history
         for element in pose_history:
-            self.last_poses[element] = pose_history[element].poses_dictionary[list(pose_history[element].poses_dictionary.keys())[-1]]
+            poses_dict = pose_history[element].poses_dictionary
+            self.last_poses[element] = poses_dict[next(reversed(poses_dict))] if poses_dict else None
 
     def add_last_poses(self, last_poses : dict[str, DFPose]):
         """