[bug-fix] Fix null ref exception in the demo recorder (#5274)

* Fix null ref exception in the demo recorder

* Update changelog

* Clearer logic

com.unity.ml-agents/CHANGELOG.md

 or actuators on your system. (#5194)
 - Removed additional memory allocations that were occurring due to assert messages and iterating of DemonstrationRecorders. (#5246)
 - Fixed a bug where agent trying to access unintialized fields when creating a new RayPerceptionSensorComponent on an agent. (#5261)
+- Fixed a bug where the DemonstrationRecorder would throw a null reference exception if Num Steps To Record was > 0 and Record was turned off. (#5274)
 
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Fixed a bug where --results-dir has no effect. (#5269)

com.unity.ml-agents/Runtime/Demonstrations/DemonstrationRecorder.cs

 
         void Update()
         {
-            if (Record)
+            if (!Record)
-                LazyInitialize();
+                return;
+            LazyInitialize();
+
+            // Quit when num steps to record is reached
             if (NumStepsToRecord > 0 && m_DemoWriter.NumSteps >= NumStepsToRecord)
             {
                 Application.Quit(0);