Fix non-modifiable cubemap data

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

 namespace UnityEditor.ShaderGraph
 {
     [Serializable]
-    public class SerializableCubemap
+    public class SerializableCubemap : ISerializationCallbackReceiver
     {
         [SerializeField]
         private string m_SerializedCubemap;
             public Cubemap cubemap;
         }
 
+        Cubemap m_Cubemap;
+
-                if (string.IsNullOrEmpty(m_SerializedCubemap))
-                    return null;
-
-                var cube = new CubemapHelper();
-                EditorJsonUtility.FromJsonOverwrite(m_SerializedCubemap, cube);
-                return cube.cubemap;
+                if (m_Cubemap == null && !string.IsNullOrEmpty(m_SerializedCubemap))
+                {
+                    var cube = new CubemapHelper();
+                    EditorJsonUtility.FromJsonOverwrite(m_SerializedCubemap, cube);
+                    m_Cubemap = cube.cubemap;
+                    m_SerializedCubemap = null;
+                }
+                return m_Cubemap;
-            set
-            {
-                if (cubemap == value)
-                    return;
+            set { m_Cubemap = value; }
+        }
-                var cube = new CubemapHelper();
-                cube.cubemap = value;
-                m_SerializedCubemap = EditorJsonUtility.ToJson(cube, true);
-            }
+        public void OnBeforeSerialize()
+        {
+            var cube = new CubemapHelper { cubemap = cubemap };
+            m_SerializedCubemap = EditorJsonUtility.ToJson(cube, true);
+        }
+
+        public void OnAfterDeserialize()
+        {
         }
     }
 }

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Data/Util/PropertyCollector.cs

                     result.Add(textureInfo);
                 }
             }
+
+            foreach (var prop in m_Properties.OfType<CubemapShaderProperty>())
+            {
+                if (prop.referenceName != null)
+                {
+                    var textureInfo = new TextureInfo
+                    {
+                        name = prop.referenceName,
+                        textureId = prop.value.cubemap != null ? prop.value.cubemap.GetInstanceID() : 0,
+                        modifiable = prop.modifiable
+                    };
+                    result.Add(textureInfo);
+                }
+            }
             return result;
         }
     }