fix unregister agents

com.unity.ml-agents.extensions/Runtime/Teams/BaseTeamManager.cs

         public void Dispose()
         {
             Academy.Instance.PostAgentAct -= _ManagerStep;
-            foreach (var agent in m_Agents)
+            while (m_Agents.Count > 0)
-                agent.UnregisterFromTeamManager -= UnregisterAgent;
+                UnregisterAgent(m_Agents[0]);
             }
         }
 
         {
             if (!m_Agents.Contains(agent))
             {
+                agent.SetTeamManager(this);
-                agent.SetTeamManager(this);
             }
         }
 

com.unity.ml-agents/Runtime/Agent.cs

 
         internal void SetTeamManager(ITeamManager teamManager)
         {
+            // unregister current TeamManager if this agent has been assigned one before
+
             m_TeamManagerID = teamManager.GetId();
         }
     }