Fix NullReferenceException for Behavior Parameters without Agent (#5382)

* check agent existence. add warning box

* changelog

com.unity.ml-agents/CHANGELOG.md

 ### Major Changes
 ### Minor Changes
 #### com.unity.ml-agents / com.unity.ml-agents.extensions (C#)
+- Fixed NullReferenceException when adding Behavior Parameters with no Agent. (#5382)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
 - Added a fully connected visual encoder for environments with very small image inputs. (#5351)
 ### Bug Fixes

com.unity.ml-agents/Editor/BehaviorParametersEditor.cs

             so.Update();
             bool needPolicyUpdate; // Whether the name, model, inference device, or BehaviorType changed.
 
+            var behaviorParameters = (BehaviorParameters)target;
+            var agent = behaviorParameters.gameObject.GetComponent<Agent>();
+            if (agent == null)
+            {
+                EditorGUILayout.HelpBox(
+                    "No Agent is associated with this Behavior Parameters. Attach an Agent to " +
+                    "this GameObject to configure your Agent with these behavior parameters.",
+                    MessageType.Warning);
+            }
+
             // Drawing the Behavior Parameters
             EditorGUI.indentLevel++;
             EditorGUI.BeginChangeCheck(); // global
             // Grab the sensor components, since we need them to determine the observation sizes.
             // TODO make these methods of BehaviorParameters
             var agent = behaviorParameters.gameObject.GetComponent<Agent>();
+            if (agent == null)
+            {
+                return;
+            }
             agent.sensors = new List<ISensor>();
             agent.InitializeSensors();
             var sensors = agent.sensors.ToArray();