Fix minor issue with PreviousPropertyBinder (bis) (#31)

* Fix minor issue with VFXPreviousPositionBinder.cs

* Fix exception in OnEnable

Note : I didn't try it locally yet.

* Actual fix of VFXPreviousPositionBinder.cs

Co-authored-by: Julien Fryer <[email protected]>

Author: 2 people

com.unity.visualeffectgraph/CHANGELOG.md

@@ -116,6 +116,7 @@ The version number for this package has increased due to a version update of a r
 - Fix issue with strips outputs that could cause too many vertices to be renderered
 - SpawnIndex attribute returns correct value in update and outputs contexts
 - Disable Reset option in context menu for all VFXObject [Case 1251519](https://issuetracker.unity3d.com/product/unity/issues/guid/1251519/) & [Case 1251533](https://issuetracker.unity3d.com/product/unity/issues/guid/1251533/)
+- Avoid other NullReferenceException using property binders
 
 ## [7.1.1] - 2019-09-05
 ### Added

com.unity.visualeffectgraph/Runtime/Utilities/PropertyBinding/Implementation/VFXPreviousPositionBinder.cs

@@ -14,7 +14,7 @@ class VFXPreviousPositionBinder : VFXBinderBase
         protected override void OnEnable()
         {
             base.OnEnable();
-            oldPosition = Target.position;
+            oldPosition = Target != null ? Target.position : Vector3.zero;
         }
 
         public override bool IsValid(VisualEffect component)