Fixed a nullref on undo parameters when graph is closed

Author: alelievr

Assets/com.alelievr.NodeGraphProcessor/Editor/Views/ExposedParameterView.cs

@@ -103,6 +103,7 @@ protected override void Initialize(BaseGraphView graphView)
             RegisterCallback<DragUpdatedEvent>(OnDragUpdatedEvent);
             RegisterCallback<DragPerformEvent>(OnDragPerformEvent);
             RegisterCallback<MouseDownEvent>(OnMouseDownEvent, TrickleDown.TrickleDown);
+            RegisterCallback<DetachFromPanelEvent>(OnViewClosed);
 
             UpdateParameterList();
 
@@ -112,6 +113,9 @@ protected override void Initialize(BaseGraphView graphView)
             });
         }
 
+        void OnViewClosed(DetachFromPanelEvent evt)
+            => Undo.undoRedoPerformed -= UpdateParameterList;
+
         void OnMouseDownEvent(MouseDownEvent evt)
         {
             blackboardLayouts = content.Children().Select(c => c.layout).ToList();