[Material graph] Fix up subgraph asset reference serialization.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Testing/IntegrationTests/Graphs/WithSubGraph.ShaderGraph

   m_Enabled: 1
   m_EditorHideFlags: 0
   m_Script: {fileID: 11500000, guid: 562bd27a5e73fc64d80a1d9d936ffbd5, type: 3}
-  m_Name: New Shader Graph 1
+  m_Name: WithSubGraph
   m_EditorClassIdentifier: 
   m_MaterialGraph:
     m_MaterialOptions:
           \       \\\"y\\\": 0.0,\\n        \\\"z\\\": 0.0,\\n        \\\"w\\\": 0.0\\n
           \   },\\n    \\\"m_ConcreteValueType\\\": 2,\\n    \\\"m_ShaderOutputName\\\":
           \\\"UV\\\"\\n}\"\n        }\n    ],\n    \"m_OutputPrecision\": 1,\n    \"m_PropertyName\":
-          \"\",\n    \"m_Description\": \"\",\n    \"m_Exposed\": 1,\n    \"m_TextureGuid\":
-          \"76c3d794167d8524481195736f067518\",\n    \"m_TextureType\": 0\n}"
+          \"\",\n    \"m_Description\": \"\",\n    \"m_Exposed\": 1,\n    \"m_SerializedTexture\":
+          \"{\\n    \\\"texture\\\": {\\n        \\\"fileID\\\": 2800000,\\n        \\\"guid\\\":
+          \\\"76c3d794167d8524481195736f067518\\\",\\n        \\\"type\\\": 3\\n    }\\n}\",\n
+          \   \"m_TextureType\": 0\n}"
       - typeInfo:
           fullName: UnityEngine.MaterialGraph.Vector1Node
           assemblyName: Assembly-CSharp
           \       \\\"x\\\": 0.0,\\n        \\\"y\\\": 0.0,\\n        \\\"z\\\": 0.0,\\n
           \       \\\"w\\\": 0.0\\n    },\\n    \\\"m_ConcreteValueType\\\": 4,\\n
           \   \\\"m_ShaderOutputName\\\": \\\"Output1\\\"\\n}\"\n        }\n    ],\n
-          \   \"m_OutputPrecision\": 1,\n    \"m_SubGraphAssetGuid\": \"63179f4461979734ba87324ec6b1268d\"\n}"
+          \   \"m_OutputPrecision\": 1,\n    \"m_SerializedSubGraph\": \"{\\n    \\\"subGraph\\\":
+          {\\n        \\\"fileID\\\": 11400000,\\n        \\\"guid\\\": \\\"63179f4461979734ba87324ec6b1268d\\\",\\n
+          \       \\\"type\\\": 2\\n    }\\n}\",\n    \"m_SubGraphAssetGuid\": \"\",\n
+          \   \"serializedVersion\": 1\n}"
       - typeInfo:
           fullName: UnityEngine.MaterialGraph.TextureNode
           assemblyName: Assembly-CSharp
           \       \\\"y\\\": 0.0,\\n        \\\"z\\\": 0.0,\\n        \\\"w\\\": 0.0\\n
           \   },\\n    \\\"m_ConcreteValueType\\\": 2,\\n    \\\"m_ShaderOutputName\\\":
           \\\"UV\\\"\\n}\"\n        }\n    ],\n    \"m_OutputPrecision\": 1,\n    \"m_PropertyName\":
-          \"\",\n    \"m_Description\": \"\",\n    \"m_Exposed\": 1,\n    \"m_TextureGuid\":
-          \"5a5e07ee958edbd468227e1d9ac4726a\",\n    \"m_TextureType\": 3\n}"
+          \"\",\n    \"m_Description\": \"\",\n    \"m_Exposed\": 1,\n    \"m_SerializedTexture\":
+          \"{\\n    \\\"texture\\\": {\\n        \\\"fileID\\\": 2800000,\\n        \\\"guid\\\":
+          \\\"65e6d39bbd36f9c45a618111370cb870\\\",\\n        \\\"type\\\": 3\\n    }\\n}\",\n
+          \   \"m_TextureType\": 3\n}"
       - typeInfo:
           fullName: UnityEngine.MaterialGraph.ColorNode
           assemblyName: Assembly-CSharp
           \       \\\"x\\\": 0.0,\\n        \\\"y\\\": 0.0,\\n        \\\"z\\\": 0.0,\\n
           \       \\\"w\\\": 0.0\\n    },\\n    \\\"m_ConcreteValueType\\\": 4,\\n
           \   \\\"m_ShaderOutputName\\\": \\\"Output1\\\"\\n}\"\n        }\n    ],\n
-          \   \"m_OutputPrecision\": 1,\n    \"m_SubGraphAssetGuid\": \"10c41d675e06a124a845154dccbcafa8\"\n}"
+          \   \"m_OutputPrecision\": 1,\n    \"m_SerializedSubGraph\": \"{\\n    \\\"subGraph\\\":
+          {\\n        \\\"fileID\\\": 11400000,\\n        \\\"guid\\\": \\\"10c41d675e06a124a845154dccbcafa8\\\",\\n
+          \       \\\"type\\\": 2\\n    }\\n}\",\n    \"m_SubGraphAssetGuid\": \"\",\n
+          \   \"serializedVersion\": 1\n}"
       m_SerializableEdges:
       - typeInfo:
           fullName: UnityEngine.Graphing.Edge

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Testing/IntegrationTests/Graphs/cracks.ShaderSubGraph

         0.0,\\n        \\\"w\\\": 0.0\\n    },\\n    \\\"m_ConcreteValueType\\\":
         2,\\n    \\\"m_ShaderOutputName\\\": \\\"UV\\\"\\n}\"\n        }\n    ],\n
         \   \"m_OutputPrecision\": 1,\n    \"m_PropertyName\": \"\",\n    \"m_Description\":
-        \"\",\n    \"m_Exposed\": 1,\n    \"m_TextureGuid\": \"6e8d12f68bae2294da814f9d4c81b29a\",\n
-        \   \"m_TextureType\": 0\n}"
+        \"\",\n    \"m_Exposed\": 1,\n    \"m_SerializedTexture\": \"{\\n    \\\"texture\\\":
+        {\\n        \\\"fileID\\\": 2800000,\\n        \\\"guid\\\": \\\"6e8d12f68bae2294da814f9d4c81b29a\\\",\\n
+        \       \\\"type\\\": 3\\n    }\\n}\",\n    \"m_TextureType\": 0\n}"
     - typeInfo:
         fullName: UnityEngine.MaterialGraph.SubtractNode
         assemblyName: Assembly-CSharp

MaterialGraphProject/Assets/UnityShaderEditor/Runtime/SubGraph/SubGraphNode.cs

+using System;
+#if UNITY_EDITOR
+#endif
 using UnityEngine.Graphing;
 
 namespace UnityEngine.MaterialGraph
         , IOnAssetEnabled
     {
         [SerializeField]
-        private string m_SubGraphAssetGuid;
+        private string m_SerializedSubGraph = string.Empty;
+
+        [Serializable]
+        private class SubGraphHelper
+        {
+            public MaterialSubGraphAsset subGraph;
+        }
+
 
 #if UNITY_EDITOR
         public MaterialSubGraphAsset subGraphAsset
-                if (string.IsNullOrEmpty(m_SubGraphAssetGuid))
-                    return null;
+
+                if (serializedVersion < kCurrentSerializedVersion)
+                    DoUpgrade();
-                var path = AssetDatabase.GUIDToAssetPath(m_SubGraphAssetGuid);
-                if (string.IsNullOrEmpty(path))
+                if (string.IsNullOrEmpty(m_SerializedSubGraph))
-                return AssetDatabase.LoadAssetAtPath<MaterialSubGraphAsset>(path);
+                var helper = new SubGraphHelper();
+                EditorJsonUtility.FromJsonOverwrite(m_SerializedSubGraph, helper);
+                return helper.subGraph;
-                var assetPath = AssetDatabase.GetAssetPath(value);
-                if (string.IsNullOrEmpty(assetPath))
-                    return;
-
-                m_SubGraphAssetGuid = AssetDatabase.AssetPathToGUID(assetPath);
+                var helper = new SubGraphHelper();
+                helper.subGraph = value;
+                m_SerializedSubGraph = EditorJsonUtility.ToJson(helper, true);
-#else
-        public MaterialSubGraphAsset subGraphAsset
+
+       // SAVED FOR LATER
+        [SerializeField]
+        private string m_SubGraphAssetGuid;
+
+        [SerializeField]
+        private int serializedVersion = 0;
+        const int kCurrentSerializedVersion = 1;
+        
+        private void DoUpgrade()
-            get
-            {
-                return null;
-            }
-            set
-            {}
+            var helper = new SubGraphHelper();
+            if (string.IsNullOrEmpty(m_SubGraphAssetGuid))
+                helper.subGraph = null;
+
+            var path = AssetDatabase.GUIDToAssetPath(m_SubGraphAssetGuid);
+            if (string.IsNullOrEmpty(path))
+                helper.subGraph = null;
+
+            helper.subGraph = AssetDatabase.LoadAssetAtPath<MaterialSubGraphAsset>(path);
+
+            m_SerializedSubGraph = EditorJsonUtility.ToJson(helper, true);
+            serializedVersion = kCurrentSerializedVersion;
+            m_SubGraphAssetGuid = string.Empty;
+#else
+        public MaterialSubGraphAsset subGraphAsset {get; set:}
 #endif
 
         private SubGraph subGraph