Fix Mouse Event Binder in player (#175)

* Fix missing call to CreateVFXEventAttribute

OnValidate is only called in editor, in runtime, we should init cache data with OnEnable

* *Update changelog.md

Author: PaulDemeulenaere

com.unity.visualeffectgraph/CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Prevent creation of subgraph containing only partial systems [Case 1284053](https://issuetracker.unity3d.com/product/unity/issues/guid/1284053/)
 - Fix [Case 1223747](https://fogbugz.unity3d.com/f/cases/1223747/)
 - Prevent pasting context within operator/block subgraph [Case 1235269](https://issuetracker.unity3d.com/product/unity/issues/guid/1235269/)
+- VFXEventBinderBase throwing a null reference exception in runtime
 
 ## [10.2.0] - 2020-10-19
 ### Added

com.unity.visualeffectgraph/Runtime/Utilities/EventBinding/VFXEventBinderBase.cs

@@ -13,7 +13,17 @@ abstract class VFXEventBinderBase : MonoBehaviour
         [SerializeField, HideInInspector]
         protected VFXEventAttribute eventAttribute;
 
+        protected void OnEnable()
+        {
+            UpdateCacheEventAttribute();
+        }
+
         private void OnValidate()
+        {
+            UpdateCacheEventAttribute();
+        }
+
+        private void UpdateCacheEventAttribute()
         {
             if (target != null)
                 eventAttribute = target.CreateVFXEventAttribute();