C# How to access gameObject in main thread

get_gameObject can only be called from the main thread.

is the error I am getting which I've figured out is to do with the way event handlers are implemented in C#.

In my Start method I have

   EventManager.Instance.AddListener(EVENT_TYPE.UPDATE, this);

Then in my OnEvent handler method I have

 public void OnEvent(EVENT_TYPE Event_Type, Component Sender, object Param=null)
     {
         //Detect event type
         switch (Event_Type)
         {
             case EVENT_TYPE.UPDATE:
                Debug.Log(" UPDATE Timer event received Param = "+ Param);
                 foo(Param as string);
                 break;
         }
     }

And then my foo method

  private void foo(string param)
     {
         Debug.Log("ASSERT foo gameObject" + gameObject);

So gameObject is null because the foo method is not on the main thread. So how DO I access gameObject then?