Checking if an object is null, results in NRE

     void OnTriggerEnter(Collider col)
     {
         if (m.Target == null)
         {
             if (col.gameObject.tag == "Player")
             {
                 Debug.Log("found player");
                 m.Target = col.transform;
             }
         }
     }

So I have this code where I want to set a player as a target as long as there is no target. Target is a Transform.

The problem I have here is that it gives me NullReferenceExceptions on the check.

So what do you guys think as a possible solution. How to do this?