fixed opponent setting

Project/Assets/ML-Agents/Examples/Tennis/Scripts/HitWall.cs

         m_Area = areaObject.GetComponent<TennisArea>();
         m_AgentA = m_Area.agentA.GetComponent<TennisAgent>();
         m_AgentB = m_Area.agentB.GetComponent<TennisAgent>();
+        m_AgentA.OpponentRb = m_AgentB.GetComponent<Rigidbody>();
+        m_AgentB.OpponentRb = m_AgentA.GetComponent<Rigidbody>();
     }
 
     public void ResetPoint()

Project/Assets/ML-Agents/Examples/Tennis/Scripts/TennisAgent.cs

 {
     [Header("Specific to Tennis")]
     public GameObject ball;
-    public GameObject opponent;
+    [HideInInspector]
+    public Rigidbody OpponentRb;
+
-    Rigidbody m_OpponentRb;
     HitWall m_BallScript;
     TennisArea m_Area;
     float m_InvertMult;
         m_Existential = 1f / MaxStep;
         m_AgentRb = GetComponent<Rigidbody>();
         m_BallRb = ball.GetComponent<Rigidbody>();
-        m_OpponentRb = opponent.GetComponent<Rigidbody>();
         m_BallScript = ball.GetComponent<HitWall>();
         m_Area = myArea.GetComponent<TennisArea>();
         var canvas = GameObject.Find(k_CanvasName);
 
         //sensor.AddObservation(m_InvertMult * (opponent.transform.position.x - myArea.transform.position.x) / -25f);
         //sensor.AddObservation((opponent.transform.position.y - myArea.transform.position.y) / -7f);
-        //sensor.AddObservation(m_InvertMult * m_OpponentRb.velocity.x / 20f);
-        //sensor.AddObservation(m_OpponentRb.velocity.y / 20f);
+        //sensor.AddObservation(m_InvertMult * OpponentRb.velocity.x / 20f);
+        //sensor.AddObservation(OpponentRb.velocity.y / 20f);
 
         sensor.AddObservation(m_InvertMult * (transform.position.x - myArea.transform.position.x));
         sensor.AddObservation(transform.position.y - myArea.transform.position.y);
         sensor.AddObservation(m_InvertMult * m_BallRb.velocity.x);
         sensor.AddObservation(m_BallRb.velocity.y);
 
-        sensor.AddObservation(m_InvertMult * (opponent.transform.position.x - myArea.transform.position.x));
-        sensor.AddObservation(opponent.transform.position.y - myArea.transform.position.y);
-        sensor.AddObservation(m_InvertMult * m_OpponentRb.velocity.x);
-        sensor.AddObservation(m_OpponentRb.velocity.y);
+        sensor.AddObservation(m_InvertMult * (OpponentRb.position.x - myArea.transform.position.x));
+        sensor.AddObservation(OpponentRb.position.y - myArea.transform.position.y);
+        sensor.AddObservation(m_InvertMult * OpponentRb.velocity.x);
+        sensor.AddObservation(OpponentRb.velocity.y);
 
         sensor.AddObservation(m_InvertMult * gameObject.transform.rotation.z);
         //sensor.AddObservation((m_InvertMult * (gameObject.transform.rotation.eulerAngles.z - (1f - m_InvertMult) * 180f) - 35f) / 125f);