fix small error in pybullet-coffee; able to reproduce the assert self…

…._held_obj_to_base_link when there are more than one tasks

predicators/envs/pybullet_coffee.py

@@ -444,7 +444,8 @@ def _reset_state(self, state: State) -> None:
 
         # Create liquid in cups.
         for liquid_id in self._cup_to_liquid_id.values():
-            p.removeBody(liquid_id, physicsClientId=self._physics_client_id)
+            if liquid_id is not None:
+                p.removeBody(liquid_id, physicsClientId=self._physics_client_id)
         self._cup_to_liquid_id.clear()
 
         for cup in state.get_objects(self._cup_type):