AcademyStepper to DontDestroyOnLoad (#3789)

* AcademyStepper to DontDestroyOnLoad

* Adding a try-catch around DontDestroyOnLoad because it cannot be used in Editor Tests

Project/Assets/ML-Agents/Examples/Basic/Scripts/BasicController.cs

 using UnityEngine;
+using UnityEngine.SceneManagement;
 using MLAgents;
 
 /// <summary>
 
     Agent m_Agent;
 
+
-        ResetAgent();
+        m_Position = 10;
+        transform.position = new Vector3(m_Position - 10f, 0f, 0f);
+        smallGoal.transform.position = new Vector3(k_SmallGoalPosition - 10f, 0f, 0f);
+        largeGoal.transform.position = new Vector3(k_LargeGoalPosition - 10f, 0f, 0f);
     }
 
     /// <summary>
     }
 
     public void ResetAgent()
-    {
-        m_Position = 10;
-        smallGoal.transform.position = new Vector3(k_SmallGoalPosition - 10f, 0f, 0f);
-        largeGoal.transform.position = new Vector3(k_LargeGoalPosition - 10f, 0f, 0f);
+    {   
+        // This is a very inefficient way to reset the scene. Used here for testing.
+        SceneManager.LoadScene(SceneManager.GetActiveScene().name);
+        m_Agent = null; // LoadScene only takes effect at the next Update.
+        // We set the Agent to null to avoid using the Agent before the reload
     }
 
     public void FixedUpdate()
 
     void WaitTimeInference()
     {
+        if (m_Agent == null)
+        {
+            return;
+        }
-            m_Agent.RequestDecision();
+            m_Agent?.RequestDecision();
         }
         else
         {
                 ApplyAction(m_Agent.GetAction());
 
                 m_TimeSinceDecision = 0f;
-                m_Agent.RequestDecision();
+                m_Agent?.RequestDecision();
             }
             else
             {

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

             // Don't show this object in the hierarchy
             m_StepperObject.hideFlags = HideFlags.HideInHierarchy;
             m_FixedUpdateStepper = m_StepperObject.AddComponent<AcademyFixedUpdateStepper>();
+            try
+            {
+                // This try-catch is because DontDestroyOnLoad cannot be used in Editor Tests
+                GameObject.DontDestroyOnLoad(m_StepperObject);
+            }
+            catch {}
         }
 
         /// <summary>

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

 
         void NotifyAgentDone(DoneReason doneReason)
         {
+            if (m_Info.done)
+            {
+                // The Agent was already marked as Done and should not be notified again
+                return;
+            }
             m_Info.episodeId = m_EpisodeId;
             m_Info.reward = m_Reward;
             m_Info.done = true;