Move "convert to subgraph" to context menu

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/SearchWindowProvider.cs

 using System.Reflection;
 using UnityEditor.Experimental.UIElements;
 using UnityEditor.Experimental.UIElements.GraphView;
-using UnityEditor.Graphing;
 using UnityEngine;
 using UnityEngine.Experimental.UIElements;
 using INode = UnityEditor.Graphing.INode;
         const string k_ConvertToInlineNode = "Convert To Inline Node";
         const string k_ConvertToSubgraph = "Convert To Sub-graph";
         const string k_CopyShader = "Copy Shader To Clipboard";
+
+        public Action onConvertToSubgraphClick { get; set; }
 
         public void Initialize(EditorWindow editorWindow, AbstractMaterialGraph graph, GraphView graphView)
         {
             };
 
             // Add in contextual node actions
-            var selection = m_GraphView.selection.OfType<MaterialNodeView>().ToList();
+            var selection = m_GraphView.selection.OfType<MaterialNodeView>().Where(v => v.node != null).ToList();
             if (selection.Any())
                 tree.Add(new SearchTreeEntry(new GUIContent(k_ConvertToSubgraph, m_Icon)) { level = 1 });
             if (selection.Any(v => v.node is IPropertyFromNode))
 
         bool OnConvertToSubgraph()
         {
+            if (onConvertToSubgraphClick != null) onConvertToSubgraphClick();
             return true;
         }
 

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

         SearchWindowProvider m_SearchWindowProvider;
 
         public Action onUpdateAssetClick { get; set; }
-        public Action onConvertToSubgraphClick { get; set; }
+
+        public Action onConvertToSubgraphClick
+        {
+            get { return m_SearchWindowProvider.onConvertToSubgraphClick; }
+            set { m_SearchWindowProvider.onConvertToSubgraphClick = value; }
+        }
+
         public Action onShowInProjectClick { get; set; }
 
         public MaterialGraphView graphView
                         if (onUpdateAssetClick != null) onUpdateAssetClick();
                     }));
                 m_ToolbarView.Add(updateAssetButton);
-
-                m_ToolbarView.Add(new ToolbarSeparatorView());
-                m_ToolbarView.Add(new ToolbarSpaceView());
-                m_ToolbarView.Add(new ToolbarSeparatorView());
-
-                var convertToSubgraphButton = new ToolbarButtonView { text = "Convert to subgraph" };
-                convertToSubgraphButton.AddManipulator(new Clickable(() =>
-                    {
-                        if (onConvertToSubgraphClick != null) onConvertToSubgraphClick();
-                    }));
-                m_ToolbarView.Add(convertToSubgraphButton);
 
                 m_ToolbarView.Add(new ToolbarSeparatorView());
                 m_ToolbarView.Add(new ToolbarSpaceView());