Fix minor issues in MARL envs (metadriverse#627)

Fix issues in MARL envs

metadrive/envs/base_env.py

@@ -736,6 +736,16 @@ def agent(self):
         )
         return self.agents[DEFAULT_AGENT]
 
+    @property
+    def agents_including_just_terminated(self):
+        """
+        Return all agents that occupy some space in current environments
+        :return: Dict[agent_id:vehicle]
+        """
+        ret = self.agent_manager.active_agents
+        ret.update(self.agent_manager.just_terminated_agents)
+        return ret
+
     def setup_engine(self):
         """
         Engine setting after launching

metadrive/manager/agent_manager.py

@@ -245,7 +245,13 @@ def just_terminated_agents(self):
         return ret
 
     def get_agent(self, agent_name, raise_error=True):
-        object_name = self.agent_to_object(agent_name)
+        try:
+            object_name = self.agent_to_object(agent_name)
+        except KeyError:
+            if raise_error:
+                raise ValueError("Object {} not found!".format(object_name))
+            else:
+                return None
         if object_name in self._active_objects:
             return self._active_objects[object_name]
         elif object_name in self._dying_objects: