Fix concrete slots not being initialized.

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Graphs/IShaderGraph.cs

     public interface IShaderGraph
     {
         string GetShader(string name, GenerationMode mode, out List<PropertyCollector.TextureInfo> configuredTextures);
+        void LoadedFromDisk();
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Graphs/MaterialGraph.cs

             PreviewMode pmode;
             return GetShader(masterNode as AbstractMaterialNode, mode, name, out configuredTextures, out pmode);
         }
-         
-        public override void OnAfterDeserialize()
+
+        public void LoadedFromDisk()
-            base.OnAfterDeserialize(); 
+            OnEnable();
+            ValidateGraph();
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/SubGraph/LayeredShaderGraph.cs

 using System.IO;
 using System.Linq;
 using System.Text;
-using UnityEditor;
 using UnityEngine;
 using UnityEditor.Graphing;
 
             return finalShader.GetShaderString(0);
         }
 
+        public void LoadedFromDisk()
+        {
+            OnEnable();
+            ValidateGraph();
+        }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Importers/ShaderGraphImporter.cs

 using System;
 using System.Collections.Generic;
 using System.IO;
-using System.Linq;
 using System.Text;
 using UnityEditor;
 using UnityEditor.ShaderGraph.Drawing;
         {
             var textGraph = File.ReadAllText(path, Encoding.UTF8);
             var graph = JsonUtility.FromJson<T>(textGraph);
+            graph.LoadedFromDisk();
 
             var name = Path.GetFileNameWithoutExtension(path);
 
 
     public bool IsValidForPath(string path)
     {
-
         return
             path.EndsWith("LayeredShaderGraph", StringComparison.InvariantCultureIgnoreCase)
             || path.EndsWith("shaderGraph", StringComparison.InvariantCultureIgnoreCase);