Properly dispose of MaterialNodeViews

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Views/GraphEditorView.cs

                 node.onModified -= OnNodeChanged;
                 var nodeView = m_GraphView.nodes.ToList().OfType<MaterialNodeView>().FirstOrDefault(p => p.node != null && p.node.guid == node.guid);
                 if (nodeView != null)
+                {
+                    nodeView.Dispose();
+                }
             }
 
             foreach (var node in m_Graph.addedNodes)
             onUpdateAssetClick = null;
             onConvertToSubgraphClick = null;
             onShowInProjectClick = null;
+            if (m_GraphView != null)
+            {
+                foreach (var node in m_GraphView.Children().OfType<MaterialNodeView>())
+                    node.Dispose();
+            }
             if (m_GraphInspectorView != null) m_GraphInspectorView.Dispose();
             if (previewSystem != null)
             {